What year can I go up to that will fit an 81? Not much to chose from at the junkyard.

 

there is a ton of threads on this...
if you don't mind moving your crossmember back and redoing your front and rear driveshafts just about any G series, W-Series or the wholy grail of 4 cylinders transmissions the R-151 will work.(these transmissions are ~25" long)
if you want to do a direct bolt in replacement you need an L-50 or L-52 (about 21" long)

 

L50 and L52 are 82 and 83. 84 85 are the G series, then it goes to W (right?). But yes, G and W are slightly longer than your tranny. The L series should all bolt right up.

 

I'm just going to add that I got a 5 speed installed. It was out of a Celica and the rear mount did not line up, about 2 inches longer. And the shifter was about 3 inches back so I had to cut the floor. Over all it was worth it. The new 5 speed whines like a sports car and I guess it weighs about 100 lbs less as old 4 speed had a cast iron bell housing !