Ditto.... to Dederichs Motorsports. Joe Dederichs is the man. He'll put together the whole package for you, including harnesses, driveshaft and trans fluid, and his prices are good. He'll work with you directly and you can get him on the phone. He has lots of experience with Mustangs, and good access to Tremec. I just bought, and had installed, the XL Magnum 2.66 box with the .80 and .63 OD's, and he was saying that this box/gear combo is going to be readily available soon. My car is a road course track car and sees no street time. I went with 4.10 rear gears and a T-2R diff. Have only run about one event so far, and the gearing is good. The splits are excellent between gears. Taking some getting used to from my OE MT-82 with 3.55 rear, but I can safely tell you that the gearing will be better overall for road course. I run Sonoma, Laguna and Thunderhill. Fair warning: the 2.66 is pretty tall for initial starts - need to slip the clutch and rev a bit - but it's a good gear for rolling in the paddock, once you're rolling.
The feel of the shifter is night and day to the MT-82. Very crisp, very positive. Hard if not impossible to blow it, and really impossible to grab reverse. I had the MGW short throw installed also. This package is by far the best mod to date. After blowing two Mt-82's, this was a no brainer. Expensive, yes. But the aggravation of numb shifting, missed shifts, 4-3 instead of 4-5 mis-shifts, etc., will be gone....finally.
There are still a few minor issues to resolve, but it's a clean retrofit package. And this - one bonus I noticed - my car is running 10-20 degrees cooler. I suspect this is primarily due to less running near the rev limiter (due to the ratios) and maybe secondarily due to the trans runs cooler. I also had a trans cooler installed (protecting my investment).
Talk to Joe and good luck.