The only five speed that will bold directly into a B10 chassis with an A14, assuming that the A14 is in exactly the same position as the A10 was, would be the 56A five speed & these are pretty damn thin on the ground & fairly expensive. They're not particularly suited to use behind the larger capacity engines either as history shows us that they get abused & broken leaving owners crying about how weak they are.

What have you budgeted for a good gearbox as you might need to be prepared to go into four digit teritory.

These are not overdrive gearboxes either if that's what you need.
My 56A five speed may be coming up for sale later on, but not untill I get my option 1 gearbox home, & that might be a little while yet.