I think you've covered all your options. You either need to lower your compression or raise the octane level. You'll just have to weigh your options, and decide which way to go.
If you don't wanna mess with your heads, you'll have to use dish pistons. I think you can buy Auto Tec/Race Tec brand custom pistons for about $200 less than the Ross brand pistons. Another way to go is to bore .060 over, if your block will safely bore that much, then use std bore SP L2262F forged 400 pistons, and have a competent machine shop cut a dish in the piston top. There is plenty of meat, and this has been done lots of times.
I would not consider building with CAST pistons at all !!!! If you do get a little detonation, FORGED pistons are more likely to survive.
Icon makes a 400 piston with a 14cc dish, for just over $500 shipped. If your block will go .060 safely, and 14cc will lower you CR enuff, then these pistons could be a solution. The valve reliefs are not in the same place as in 389 pistons. But, unless you run a really high lift cam, the dish should provide plenty of valve clearance. If you know all the specs you can figure out how much dish you need to get the CR you want, using this CR calculator.
Compression Ratio Calculator - Wallace Racing
Another piston option is to bore .060 over, and use the Icon FHR IC9946-std pistons. They have 8 reliefs, for either early or late heads. And if these reliefs don't reduce compression enuff, a dish can be machined in the relief area. These are just over $400 + shipping. So, depending on machining cost, the total should be just over $500. Or, if you are handy with a hand grinder, you can dish 'em yourself. Hey, it's just an option.
If you are going to rebuild your shortblock anyhow, and are not set on using the block you have, or don't wanna bore it .060, you might consider switching to a 400 block. That way, you can just bore .030 or .040, use dish pistons, and have more cubic inches too. Hey, there are several ways to do it. Just depends on your budget & how you wanna go about it.
If you wanna keep your block, you can also buy DSS pistons, with 5cc reliefs, and either have a dish cut in 'em, or have your head chambers enlarged.
Hey, if you are not set on using your 389 heads, you can go with later heads, which have hardened seats, screw-in studs, and will reduce your compression with flat top pistons. DSS also makes a forged flat top for this app. DSS 389 pistons come in 2 bore sizes--4.080 & 4.100
Late '73 & '74 #46
small valve heads are the 1st late heads that come to mind, for this app. With these you can use an Edelbrock Performer intake, a good rebuilt Q-jet, and a small Voodoo cam. But hey, if you wanna keep your engine looking original, I understand that.
I use Torco Accelerator octane booster in my 455 bracket engine. It is one of the boosters that actually works, unlike most of the parts store boosters. It's a little over $20 a can.
The Best Fuel Additive Octane Booster Torco Accelerator | eBay