LSx Engine Decking?

[WSSix]

I doubt you'll get to 400. Remember, the LS6 only had 400 stock and it used 10.75 compression and was 5.7 liters with the difference being 100% bore size. Your 317s are basically low compression LS6 heads. I'm afraid those heads and that cam will not allow the charge to be compressed enough for a good burn. Honestly, it's beyond my knowledge but my gut and experience tells me you need minimum 10:1 with that cam on this type engine.

My suggestion would be to approach this from a different angle. I'd ask around and say what can I do to build an 87 octane engine and state what parts you have available. I'm assuming you still have the 5.3 heads. It may turn out that the best idea is to use the 5.3 heads and a different cam. I know it adds money to the build but if you miss your mark the first time, you'll be adding time on top of costs to get it right.

Also do not forget that if you are using an automatic transmission behind the engine you have tremendous tuning abilities with the drive train to make all your ponies work for you. Manual guys don't have that luxury without getting stupid expensive with gear changes in the transmission. Use the capabilities of the torque converter and electronic control of the transmission to your advantage. I'd rather go fast than have a dyno queen