There has been very few people that have really shown what a R5/P7 engine is capable of. Most guys are buying up cheap ex Nascar parts, rather than starting with new castings etc and doing a full effort build. To that end, it's not an engine that's really proven itself as a drag racing platform.
x2, the R5 block with a deck reinforcement plate will accept a rather large bore, get a set of new P7 castings, move the exhaust valve moved over to make room for a larger intake valve, and you got something to work with. The NASCAR port/valve/cam programs were never meant to feed anything more than 358 c.i.
