I have to disagree, Id rather something light weight and available
and although the BW are common and avail with many ratios they are
a heavy bugger defeating the reason to own the lightest datsuns mass produced.
I consider the B series a unique and important collectable and if you
can spend some money on this part of the build to compliment its lightweight.
I suggest an all alloy h190, modified h150 (usually strong enough for good
drivers), Alfa Romeo 105 (3.4, 3.6, 4.11, 4.3, 4.5 ratios avail) or a second
hand Winters or other similar quickchange for hotrods.
Gav since you are running one of the lightest engines ever made for cars
then please consider keeping the rest of that blue coupe featherweight.