Using the trusty Moroso Power/Speed calculator, I have his 416 @ 495 HP and your 421 @ 520 HP.

I think the INDY version of the RPM is worth some power, espesially on
a 4"+ stroker. Keith's are as big as I could get with tubed pushrods and
I think it would be faster if I could have opened them up another .100".

I'd also consider a different cam on the 416.

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