Easiest I expect would be the W50 as most if not all would have been behind R series motors to start with and so should bolt straight up to the engine. The only other thing you may need to change is the g/box cross member. This may just be either a swap between using and auto or manual cross member or just turning it around 180 degrees to line up the holes. Should be a easy fit I'd think as I imagine the RT104 would of had a W40 manual in it which is basically the same g/box but without the extra bulge for the fifth gear in the rear extension housing.
Other thing to check is the number of splines on your current g/box output / tailshaft.
If you are sourcing from a celica W50 were in all 2lt celicas up to and including RA40s
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