The T56 is a very long gearbox. I measured mine a while back and it was about 850mm from the output shaft to the face where it bolts to the engine. The tailshaft will work out to be about 17" long but that may vary depending on what your engine setback is. I would have some real concerns with having a live axle behind such a short tail shaft unless you can limit your suspension travel. IRS is definitely the go.
The price on the box seems OK if it has the ford patern bellhousing. Holden and Chev ones are more common but not easily adapted to the Ford motor. I paid $2800 for mine S/H and a friend just picked one up for about $3500 for his but it included clutch and flywheel.
What is the part number on the box? It's stamped on a little aluminium tag held under one of the extension housing bolts. There are quite a few different torque ratings for these boxes. You can reference your part number on the
TTC Webpage and find out which one you have (unless it was a preproduction "special build" like mine was
).
Cheers