LM4 has flat top pistons I'm 95% sure
Just mill the head .03 and your at 11:1 compression with stock height .051 gaskets. I would not milled heads more than this! some people will mill a max of .04 on 243s
Your piston to valve clearance should be fine but I would defiantly do some research to make sure b/c of the flat tops.
You will need to order a push rod length checker if you mill the heads. All blocks can be different so don't listen to somebody that tells you the "proper length" check for your self.
Should be around a 7.375 but like I said check!
2001 5.3L 2wd 3.42s. Pacesetter long tubes, custom 3" dual exhaust, UDP, CAI, 1.8 yella terra rockers, efans, milled .03 862, 210/218 .584/.584 116Lsa, custom hp tune. 336rwhp 351rwtq on 83lbs 20's GT KILLER