With the A15 there are a number of 5 speed transmissions that bolt right up.
Even when looking for the "60" model transmissions you are not stuck with finding certain serial numbers that start with "F" so you can get a transmission that will fit in your tunnel a lot easier and of course you don't have to buy an expensive conversion kit. You can gain power with the motor swap, but you lose originality. Also a small cut is needed in the front to accommodate the front lower pulley and the motor mounts have to be changed.
I have a downdraft carburetor on mine, but if you look around you can find the GX head and put side draft carbs on it, webers, delortos or SU's.
I picked up a "60" transmission with new clutch for $150 earlier this month.
To install it I'll buy a new drivers side motor mount and some gear oil, so probably another $50 and 5 hours with BillM's skillful help. :wink:
As far as I know my motor came out of a 1980 Datsun B210 Deluxe. That's the one I say when getting parts and so far they have been right.