The T56 Magnum is an 'over the counter' version of the TR6060. For all intents and purposes it IS a 6060. All modern Tremec transmissions have a build number stamped on a tag. For a TR6060/T56 Magnum it will look something like this "TUET1648" This will tell us what its internals are in terms of double/triple cone synchronisers, and the gear ratios. I suggest before you buy it, you verify the tag number. I would avoid the earlier T56 due to parts availability for you. The company I work for sells Tremec parts and back orders for us are in the 45-90 day territory or longer on T56 parts (TR6060 and T56 Mag are a bit easier to get but more expensive). If you get this gearbox, and it breaks something, expect there to be delays on parts for 30-60 days and be pleasantly surprised if one of us has it in stock.
You may be better off buying a newly built unit from someone like D&D Performance, Tick Performance, RPM Transmissions or The Gear Box. At least what you're getting will be newly built with Tremec parts and it will be exactly what you need, instead of some mystery box.