I have some Jegs aluminum heads on my new 383, and I dont think i'm getting quite the compression i was hoping for, so I'm thinking about having the heads milled. Should I be concerned about piston to valve clearance? The block is standard deck, the pistons are flat top with 2 valve relief. I have a comp XE268 cam with .477/.480 lift. Also, what has to be done to my intake manifold to insure proper fit? thanks.

you shouldnt have any problems.

how much are you going to mill? Flat mill should be OK...angle mill could be close. I would check it either way...better safe

You should be able to mill the crap out of the heads with that cam but it is always good practice to see what kind of room you have ;)

How much are you talking about here??

The intake surfaces of the heads should be corrected when the heads are cut not the intake manifold.
If you were cut a lot, the bottom of the intake or top of the block may need cutting but we are talking a LOT.