It's a 1973 400 block that's .10 over. I think the previous owner put a mild cam in it and i can't imagine it's doing that much with the small valves and low compression. It's running a quadrajet.
Not sure there's that much else to report on this one?
There's a set of the smaller volume 6x's with the larger valves on craigs list locally for $400...
OK, more info. The 4x heads can either be pressed-in studs or screw-in studs depending on when they were cast. IF you have the screw-in studs, I would keep the heads and have the larger intake valves installed to save some money. Read this: 4X Racing Heads - Tech Articles - Hot Rod Network
The 6x's have different chamber volumes from 93cc to 124cc, so make sure you are getting the smaller chambers. Here is a guide to chamber size: Pontiac V8 Cylinder Heads
Keep in mind that I would not simply buy the heads and expect to bolt them on and go -unless advertised as being completely rebuilt. If you buy them, first thing I would do is have them disassembled and magnafluxed for cracks - make sure you get in writing from the seller a money back guarantee if the heads are cracked or unusable.
If they pass, then I would have them rebuilt, ie resurface the heads, new valves, 3-angle valve job, hardened exhaust seats if desired (I don't feel it is necessary if going stainless steel valves), new exhaust valves, springs, retainers, keepers, & seals. Install new 7/16" ARP rocker arm studs to replace the factory 3/8" screw-in bottle neck studs (which will require polylock rocker arm nuts), install a new water nipple outlet at this time and new welch plugs.
However, your 4X heads should have 98cc chambers which will match the chamber size of the 6X with 100cc's if that is what is being offered. If they are the 93cc chamber 6X heads, then I would go that route just for that reason alone.
But if your 4X heads match the 6X 100cc heads, I might want to think on it a little. You can have 7/16" screw-in studs installed(polylock rocker arm nuts will be needed). Have the heads resurfaced. Install the bigger 2.11" intakes and have a 3 angle valve job, hardened valve seats on the exhaust side if desired, new exhaust valves, springs, retainers, keepers, and seals. New welch plugs & heater nipple(if yours looks at all rusty).
So if you compare the work needed to upgrade the 4X heads versus purchasing the 6X heads - you will see much of the same parts are going to be needed. The difference I see is fitting the 2.11" intakes and adding the 7/16" screw-in rocker arm studs as being the extra cost items. That is where I would visit my local machine shop and get a price on the costs and measure them against the purchase price of the 6X heads. I would think you should find a few dollars being saved and I would use the savings to put back into the rebuild of the 4X heads.
Never hurts to pick up a backup set of heads either - I have a couple sets. You can have them worked until you are ready to install them.
Power gains? Probably more noticeable in the mid to upper RPM range providing you have a good flowing exhaust system. Screw-in rocker arm studs allow you to go with a more radical cam profile without worry of pulling up/out the press-in studs. With a low compression engine as yours, I like the Comp Cams Xtreme cams ground on the 110 LSA. This is what I would install to wake up the engine if I did not know what cam was in the engine now. It may very well be a Comp Cams cam as they are often used in Pontiac builds. Used one of these cam kits (matched valve springs with the cam/lifters) in my '72 400CI 7K3 head build with low compression and was real happy with the performance of the cam. The 110 LSA builds cylinder pressure so it helps the already low 8.0 compression ratio.
So a few things to consider. I think I covered it all, but use this as a guide rather than gospel.