It is kinda hard to suggest a piston for any combo without knowing some exact numbers. Most piston choices will be forged, and you can get differing out-of-the-catalog over sized pistons depending on how much the block may need to be bored to clean it up - typically .030" over, then .040", then .060".
Your best bet to run on the 90 octane gas would be to shoot for 9.0 compression. You can go higher, but you may be taking a chance with engine detonation, but some feel comfortable going higher and you will read about guys running 10.0 on pump gas which to me is skeptical, but doable if timing is adjusted and everything is in perfect tune.
Things you need to know are specific values that will determine the compression ratio. You will need to know exactly what your head chamber cc's are, not the numbers they are supposed to be or what the internet says they are. Actual head chamber volume can vary from advertised numbers. Head chamber volumes can change by grinding in the chambers to clean-up/improve flow, head milling, the height of the valve as it sits on the valve seat, or even the design of the valve head. That said, you have your heads cc'd @ 93.5.
You will need to know what the deck height is, ie the top of the cylinder measured to the top of the piston. This is the space above the piston and has volume in cc's. Typically a Pontiac piston will sit .015-.020" down in the bore. A block can be milled down to bring the top of the piston flush - called zero deck height. I don't recommend it, but others will. I'll throw off head/intake angle geometry and the intake side surfaces have to be milled accordingly to match any angle changes. I'd rather see a custom piston with the wrist pin relocated, if anything., but have not heard of this being done, so who knows. The 455, .030" over @ .020" down in the bore will calculate to 4.5 cc's as the cylinder volume above the piston (some pistons are made with a piston edge that is beveled which will add additional cc's in an effort to further reduce compression).
Your head gasket, that hole in the center, has a volume in cc's. The thickness of the gasket used can be used as an adjustment to raise/lower compression slightly. The Felpro gasket, as most will list, is supposedly .039" when compressed. For example, 455 .030" over (4.180") with .039" compressed head gasket is 8.7 cc's. Using a thinner .027" compressed gasket is 6 cc's.
So now with these numbers above, you can calculate for your piston and determine the amount of cc's that has to be built into the piston by either the valve relief notches or a dished area cast/milled into the tops of the pistons. So let's use 93.5 + 4.5 + 8.7 = 106.7 as your total number of cc's above the piston.
Using the Wallace Compression Calculator, Compression Ratio Calculator - Wallace Racing
, you can plug in the numbers. I used the 455 .030" bore, 4.180", as the Gasket Bore Diameter which will be fairly close. Used .020" for Deck Height. I also used the 6.6 Valve Relief/Dome cc's as a baseline for the compression ratio - which is what you adjust to change your compression.
So, plugging in the numbers, I get a compression ratio of 9.37. Not bad.
You can use a number of different pistons, but in my 455 build I used the Keith Black ICONN pistons which come in 3 different valve relief configurations, 5.5, 11, & 17 cc's. Plugging in the 5.5 cc's in place of the 6.6 cc's and the compression is now 9.44. Getting up into the compression where 90 octane might not be too comfortable.
Plugging in the 11 cc's (which I went with) and the compression is 9.04. Right were you want it.
Pluggin in the 17 cc's and the compression drops to 8.65. Not necessarily where you might want to be BUT, there are options with this ratio.
There is this thing called "Quench Area," the area between the piston and the head when the piston is at top dead center. Many builders like to see this area have a distance of .040 - .045" (depending on piston material used) to maximize the cylinder burning. The tighter measurement can aide in preventing detonation and can sometimes allow for a higher compression. With a deck height of .020" plus a gasket of .039", the Quench is .059". Not really ideal. What I did in my build was go with a Cometic .027" head gasket. .020" + .027" = .047" Quench area. Changing the formula and using the .027" head gasket yields an increased compression ratio of 8.82.
The larger Quench will obviously work because that's factory - .059" - .060", but remember leaded gas was used and leaded gas suppressed detonation. Still, it works with most builds, so not a big problem. So the 9.04 compression will work, but so will the 8.82 compression with the tighter Quench which will reduce the chances of detonation.
Using the 8.65 or the 8.82 compression, you can select a cam with the narrow Lobe Separation Angle (LSA) such as the Competition Cams series with its 110 LSA. These cams have real power in a lower compression engine because they build additional cylinder pressure - essentially acting as higher compression. They are very strong cams, but will peak out in the RPM range early which may not be too much of a problem at all reading into what you want out of your engine and the lower RPM characteristics of the 455. Wider LSA cams (114-116) will provide a broader power range but not have the lower RPM punch of the tighter LSA. The wider LSA cams typically bleed off some of the compression at the lower RPM's and is why they were selected when using higher compression engines. You will pick up the lost compression as the RPM's increase.
So selecting a cam to match your compression is just as important as the compression itself. This is where the difference between Static Compression (your actual number) versus Dynamic Compression (cylinder pressure derived by the camshaft specs, namely the Intake Valve closing degrees After Bottom Dead Center (ABDC) ). This number is found on most cam manufacturers Cam Card. That said, you can use the 9.04 compression ratio or the 8.82 compression ratio to determine your Dynamic Compression by obtaining the cam manufacturers cam spec for the Intake Valve closing point at ABDC. From what I have read, and what I shot for in my 455CI build, it to keep the Dynamic Compression below 8.0. Use this calculator to get your Dynamic Compression ratio Wallace Racing: Dynamic Compression Ratio Calculator
AND, as a DISCLAIMER, because I am not an expert on this, just advising, you want to talk with your engine builder and also get cam manufacturers recommendations to zero in what you want the engine to do as the final voice in your engine build. Keep in mind that a cam that may be strong in a 400, will be milder in a 455 due to the bigger cubes. The Comp Cams XE cams, 262, 268, or 274 would be a good choice with the lower, under 9.0 compression. https://butlerperformance.com/c-1234...ppet-cams.html