Im no expert, but i think the cam would dictate how much valve travel you would have if you are worried about the valve contacting the piston. If you are using stock gt40's with stock cam and stock ratio rockers... you shouldnt have a problem, but wait around for a more experienced answer.
I've heard in several other threads about needing to have valve reliefs in the pistons. '86 had 9.2:1 compression and flat taops and 87-? had 9.0:1. I have some E-7s on my '86 and there is no problem. Some said there would be a problem and others said I'd be fine. They were on the car when I bought it. I don't think that the small difference would make too big of a difference. Someone told me in a thread I started about my 393 stroker I am building that the piston is a good 2" down the cylinder before the valves are fully open. I'd say your best bet is to talk to someone who has done the swap. I'm interested to see because I would definitely like to put a cam in my 5.0, it will be a while before I finish my stroker.
I put some milled gtp's on with a larger cam an it was no big deal to clay them. if you're going to swap them on, it only takes a few minutes to mock up one cyl an clay it. i'm glad i did cuz i way to close, .04 clearance. i just used a carbine bit an notched the pistons myself with the motor in the car.