If you want real value for money,[you did say 'cheapest'] then an A14/15 engine, combined with a 3.9 to 1, differential from a Stanza or a series 1 or 2 Bluebird, or even a 3.7 to 1 diff from a Bluebird will have you singing your Datsun's praises even more loudly. Even in stock trim, you will appreciate the additional torque that these engines produce.

More bang for buck than anything else you could name & a job that's dead easy to do.
In fact, if you can buy a whole Sunny wagon, you can use the engine, trans & radiator, & still have the front end to do an upgrade later on.
If it is one of the A15 powered wagons, then drop me a line & I will buy the differential.[It won't fit your axle housing & is smaller & weaker anyway]