The difference betweent 85/87 and the earlier TR's is the converter. The 85/87 uses the wide mouth cat (like a MCSS). There should be systems around for the regular cat. My '83 TR has a new, system on it from NAPA. I belive it is made by Walker.
2.5" is plenty for the dual muffler design used in 1982.
I actually used a system from an 86-87. I cut the flange off of the front of the y-pipe, and simply had an adapter fitting (2 1/2 to 2 1/4 if I remember correctly) welded on so I could attach the new system to the older style cat.