I've never compared early to late side-by-side, and can't recall seeing that excellent photo comparison before - thanks for posting that.

I'm struck by the width difference in the engine mounting pads/bases.

IIRC, the engine setback difference from early B to later B K-members is about 1.5"
Buddy of mine who did that swap on an early B said it played hell with the right-side Maxie exhaust manifold at the firewall.
The setback sure seems advisable for some builds, if you're sure the headers/manifolds don't hit the firewall.