I have a chance to get a 413 fairly cheap. This is a non HP 413 (single 4 bbl). I assume they used closed chambered heads...valve sizes? How far down the hole did the stock pistons go? And is there any reason why a "B" camshaft won't work in this RB engine?
Thanks!

Get some numbers off the 413, it may be a motor home engine but the B/RB cams swap.

Drop the 413 crank into your 400?

It depends what year it is. Older 413's have 2.00 intake and 1.50 exhaust valves. I believe when they went to the 516 head, the valves went to 2.08 intake and 1.60 exhaust. If it's a later truck/motorhome engine, it'll have 2.08/1.74 valves. Also if it's a '59-61 engine the crank is gonna have a funky 8 bolt flange that won't work with later transmissions and torque convertors.

Compression I'd guess is around 9.0 if it's from a car, more like 7.8 if it's from a motorhome. And B/RB camshafts are the same.