Maybe I can save you some trouble
I posted some of this the other day:
I measured the Piston to Valve clearance today....1986 block & GT40p Heads.....
I used an old 5.0 hydraulic roller lifter which I made into a "solid" lifter for checking purposes as outlined in this article here:
crudely, I attempted the clay method....
.060 on the intake valve, .095 on the exhaust.
To which Someone else posted:
"Ideal is 0.080" on the intake and 0.100" on the exhaust."
Later I have read that you CAN NOT use the flat top pistons unless you have the E6 heads because the E6 heads had the valves recessed deeper in the head...? not sure if its true but...
If you are going to go ahead with the plan, buy some thicker head gaskets it may help?
Can you get some junkyard (notched or dished) pistons? I think they might be really common? probably from 1965-1994 sbf would be a good bet...to get you away from those flat tops... Even if you get cast pistons...There is nothing at all wrong with cast pistons, they just cant handle the abuse (forced induction) of forged. Cast and hyper.. may have better expansion properties depending on usage. Also allow you to run tighter piston to wall clearances
If you flycut you will have to get them balanced to do it right, it might not be worth the bother...you may also introduce weak spots or stress risers.