"What is the best way for me to lower the static compression ?..."
If it is indeed only 9.6, I'd go with a cam change. A cam with around 290 advertised intake duration will bleed off some of the cylinder pressure, thus making the engine more pump gas friendly. This would be cheaper than $500 dish pistons. Also, if you change pistons, you'll need to have the assembly balanced again, plus new rings, hone, etc. I would assume that it would cost you around $1000 or so to change pistons.
The cheapest small cam, which I believe would lower dynamic comp, for pump gas use, is the Summit 2801. It has 288/298 adv dur. It is similar to the Pontiac "068" grind, but with more lift. I'd consider it a good all around cam, for a mild street 400, with between 9:1 & 10:1 static comp.
I'll also add that if you get up closer to the 10:1 number, with iron heads, you may need even more advertised duration, to avoid detonation. There are lots of guys who have been running 10:1 for years, without problems. But, they have long dur cams and they keep the timing and carb in tune, to avoid problems. Most engine builders recommend around 9.5 max static, to stay safe.
Would also be a good idea to enlarge the chambers slightly. Just some minor grinding and removing sharp edges, will reduce static slightly. No need for a full head porting job. Using 78cc chambers, in the Wallace calculator, brings the static down to 9.33. 80cc chambers reduces static to around 9.16.
Again, I'm using these numbers: 4.12 bore, 3.75 stroke, 6.7 valve reliefs, .039 x 4.3 Fel-Pro #1016 gaskets, & .020 deck height. So, if any of the ACTUAL numbers differ, then, obviously, the ACTUAL static comp numbers will differ from those I got. For example: If you have the Speed-Pro L2262F pistons, they have a pin height of 1.714. This will reduce the deck height by .014, over a piston with a stock pin height of 1.7. It's recorded that many Pontiac engines came with .020 or a little more deck height. So, these pistons should reduce the deck height to somewhere around .006 to .010.
Then, if the block deck has been surfaced any, that would further reduce deck height. But, if the rods have been resized, they are slightly shorter, which will increase deck height slightly. So you see, EVERYTHING must be measured, in order to ACCURATELY determine static comp.
One other option is to switch to E-85, If it is available in your area. It will handle 10.5 comp with no problem. Lots of guys have switched. So, there is lots of online E-85 info. And, there are lots of E-85 carbs on the market.
If you don't do a lot of daily driving, and you are worried that you may have a bit too much comp, you may wanna add a couple of gallons of racing gas to your pump gas. Last I checked, 108 race gas was about $10 per gal in my area. But, since pump gas is down to under $2 per gal, a gal or 2 of race gas, may be a better solution for some, than a cam, and especially a piston change.
But, if you decide to go the dish piston route, a custom Auto-Tec / Race-Tec piston will be slightly cheaper. And, you can have the pins located to provide near zero deck height. That way you can have a good quench distance, without cutting the block deck any.
