Hi Allie,
Congrats on getting a nice Datsun 210 (B310).
Yes a Datsun L20B engine can be made to fit with only a minimal amount of fabrication.
Yes the automatics are three speed, just like a 1970 Camaro with 400 horsepower. The reason it doesn't up a hill lickety-split is that you are not pushing the pedal all the way down. These Datsun are designed to run for short periods of time at full throttle, it will not hurt them. Seriously the newer (starting mid 1980s) four-speed automatic is exactly the same but the extra gear is for highway cruising and offers zero extra oomph for acceleration.
An A14 or A15 from Datsun 210 would be great. They have a ton more power, and actually get better fuel economy than the A12A engine you have (when fitted with 5-speed transmission). If you stay with the automatic, I reckon the A14 will get worse fuel economy than the engine you have.
last month there were two Datsun B310 at Pull-A-Part wrecking in Arlington up past Everett. Had the brakes and everything on them (transmission were gone though, I think you'll find the ones 'gone' were all 5-speeds). The brake drum is only $20 new at
RockAuto.com special sale while it lasts.
If you need any assistance working on it, I'm in Bellevue. Just PM me and if I have time I'd be glad to help another Datsun owner.