I would post it on Ebay or Craigslist. That is stock trans for Vega's and Monza's (and other GM H-bodies). It is not a bad transmission, it is just hard to get parts for. I replaced all the bearings in mine by taking the old bearings out and getting new style units from the local bearing house. The large diameter main shaft bearing doesnt have an interchange.
The synchros are hard to find as well. Tail shaft bushing and rear seal are easy at local FLAPS
for the top shift boot, I used a universal CV boot kit.
My recomendation is to grab a NV3500 from a 4.3 S-10 and just bolt it in. Higher torque capacity than the T5 and will bolt to the 283 easy. Since everyone and their brother is looking for the T5 for older vehicles, these are often overlooked and discarded since they have the integral SBC bellhousing. The salvage yard down the road from my place in WI would sell them for $100 complete with clutch/PP/Flywheel and master cylinder
