5 cylinder engines seem pretty interesting, would it be a difficult task for a stock car engine manufacture to engineer a design given they're use to V8s or V10s?

While what you say here is true, it doesn't tell the whole story and also ignores my point that every modern inline 4 also uses balance shafts. Yes, you could get away with not having them on an I4, unlike a I5. There have been a few production engines which did (like GM's Iron Duke and early Quad 4s) and they have reputations for being very rough engines. The other thing is that while inline fours do have perfect primary balance, they also have a gap between power cycles which is not present with five cylinders. That means inline fives have smoother power delivery.