The other problem with using the glass rb20 box is the extra weight of the car.
So its good you got the rb20 cheap. But putting the 20 box in is really the troublesome thing, in that it creates more work for a box that will just break.
(The rb20 will just be slow, so no harm done, but the box is weak and will likely break)
How long would you have to wait it get the cash to buy an r33 box? During that time you could drop the engine in and get it running(just the engine, so whatever rewiring is required) Then box simply bolts in and you can chirp second and make whooshing noises past bus stops all day.