If your shell is really 73mm (usually mountain bike specification), and the cranks are JIS taper, then that's probably what I would go with.

All you'd need would be an appropriate Shimano-type BB tool (buy, borrow, or coop?)...plug & play.

Probably want to verify those measurements, though - markings on the old BB might give a clue to confirm?

EDIT: Guess fender1 beat me to it on the tool recommendation!