better place to start is to give more information. datsun would have made more than 1 wagon in 1981 so it would be handy to know which one or which engine it is

being that you have a 210, and you have heard its a bolt in conversion. then im guessing that the wagon you are talking about is a b310 with an L20b engine

if that is the case then if you use pretty much the whole b310 front end the engine will bolt in, you wont have to move master cylinders or anything. you will have to put the clutch pedal setup out of the b310 into your car as the L series run hydrauluic clutch setups

if you use the b310 diff or if its the same as the one you have, then the b310 tailshaft should work. but then if yours is a sedan and the other car a wagon it may be different, dont know if the wheel base changes or not
Alternator wiring will have to be extended over to the other side. coil wires might also have to be changed if you mount your coil on the same side as the distributor

transmission tunnel should be long enough, crossmember from the wagon might work in your car. you will probably have to modify the shifter hole to suit the manual shifter

thats about all, if the wagon isnt a b310...then not much of what i said is relevant