If you use the 781 heads. On a 427. Only aftermkt domed KB type open chamber pistons will work and they will have to be checked for dome clearance. (not TRW/FedMog/Sealed Power/Speed Pro/Manley/2308).
The 2308 style piston is the factory style L-88, 12-1 compression design. Hits in various places and is a hollow dome piston with not much meat on the top to cut on..
781 heads respond well to larger (and longer) valves..2.19+.100 long and 1.88+.050 long..
Yes, most 496 cranks require +.250 long rods unless you are using a tall deck..