At this stage it's a bit of a 'what if' exercise but the point on the block is well taken.

The crank in question should not require further machining to fit the B-block as it was made with smaller counterweights and B-sized main journals for exactly that purpose. Plus an 8-bolt flange.

For turning it down if/when the time comes, being friends with a crank shop does occasionally have its benefits....

Thanks all for the great advice!