74 Datsun 620,
If you are looking to swap in a new bench seat the try to get the new one complete with rails, mounts, nuts and bolts. I'm putting in two 510 bucket seats in my little 320. I just reversed the seat to floor pan mounts and removed the head rest, tight fit, but comfortable.
I'd check the 620 Forum for the transmission swap and might look at the 510 Forum for some SR20DET 5 speed swaps for new transmission.