317 heads on 5.3
 

i wish think getting the heads milled 40 thousands and than use 15 thousands shim gasket. whats everyone opinion on this?

why not just mill them 25 thousandths?

You definitely can't run a 0.015 shim gasket. With the piston being out of the hole you would have a bit of a clearance problem. Personally 0.040 is the thinnest I would go.

What is the pourpose of starting with the largest CC combustion chamber. If you want more compression why noy use LS6 heads?

i would use 243 if you could find a set for a good price and i bought a 317 heads with 25000 miles on for 50 dollars off a friend that sold his 6.0 short block and gave me a good deal on the heads. i just looking to stay at the same compression but with 317 heads but flow alot better.

DON'T DO IT!!!! There would be no gain to be had really. You would lose too much compression and wouldn't gain much if anything. If you want to do something get your stock heads milled and ported and you will be much better off, might cost a few pennies more but it would be worth it.