You've got a good car there. I had a B310 with A14 & 5 speed. About 38 mpg at 80 mph with everything stock. Drove it 285,000 miles. Was still running good when I sold it.
I'll say the more stock you keep it the less trouble you will have. Could put in A15 engine. Would bolt right in. I have a 1989 Nissan Sentra with 1600cc, throttle body fuel injection and 5 speed. Is overhead cam with 3 valves per cylinder (2 intake, one exh). Very quick car, would run circles around any stock "A" series pushrod engine. Has 287,000 miles on it and got 50 mpg on a 600 mile trip a couple weeks ago. Might take some converting to get engine block to match up to older Datsun 5 speed. Would be a quick car and good mpg too.