You can use a .250 dome piston with a closed chamber head and up to a .600 lift cam, i was doing that in the mid 80's with both a 427 and a 454 engine with a stock steel shim gaskets, if the heads haven't been milled too much and that could be the problem, maybe some one milled the heads a bunch. Either way it's best to check the valve to piston clearance before final assy. you need min. .080 on the intake and .120 on the exhaust clearance. I use modeling clay for checking.
Most of the dealers sell these pistons, jegs, summit, just be sure you tell them they are for closed chamber heads.
JMO
Zip.