The BBC may be slightly shorter top to bottom and narrower across the manifolds than the RB based on the deck heights.
The RB may be a better fit to the firewall (farther back) with its front-mount distributor - the BBC is rear mount.
Where is your center link? Steering mast and linkage?
The BBC oval heads (if they're not peanut) are probably better than the RB.

Boffin Emeritus