this reply is probably a little late, but i have some of the info you were looking for.
your engine is a Z20,
its bottom end is very simmilar to the L-series engines and has the same transmission mounting bolt pattern. most datsun transmissions will fit. look for cars that have L or Z series engines and thier transmissions will be able to mount to the back of the engine, whether or not the length of the trans (and therefore your driveshaft) will be the same and if the trans crossmember will be the same will be the big questions. you are going to have to do some research. nissan makes thier engine identification easy, it is usually on a plate in the engine compartment.
the 8 plug thing was done for emissions. nissan did it in thier Z and CA series engines, ford did it in the 2.3L 4cyl. mustang.
sorry, i'm no tranny tech. i might be able to figure it out if i had the car in front of me, but i can't diagnose a transmission noise complaint without hearing it.
if two independent (from different shops) mechanics diagnosed it the same, they could be right.
again, sorry it took so long, and good luck.