Tremec differentiates the two transmissions primarily by rated torque capacity and a few other considerations. The original or plain T56 has a Tremec specified torque rating of 450 ft/lbs. The most common point of failure is the 10 spline input shaft. Tremec (and others) offer 26 spline inputs made from reportedly better steel with larger diameters. With the use of the 26 spline inputs the original T-56 can easily handle well past 600 ft/lbs of torque.
The original T-56 and its Magnum prodigy have limited gear ratio selections and three different shifter positions possible. For some later model cars the shifter location needs to approximate the end of the tail shaft. The erector set shift linkage for this position was remedied with the advent of the MagnumXL versions. The two Magnum transmission (and the earlier T-56) are typically offered with a 2.66 or a 2.97 first gear. The original version of the T-56 is no longer manufactured by Tremec although service parts are currently still available.
The original T-56 transmission has been superseded by the current T-56 Magnum that offers all the original T-56 features plus the 700 ft/lb torque rating. All input shafts on the Magnums are 26 spline. This is Tremec's web page for the transmission =>
T-56 Magnum
This is an image off the Tremec site, of a regular T-56 Magnum;
Note the rear shifter position can be rotated like a TKO to provide two shifter positions. In front of the rear shifter well is a second mid shift location to move the shifter forward a few more inches if you opt for one of the mid shift kits.
The Magnum XL is essentially the same transmission with an extended tailshaft and a rear shifter location that is at the very back of the tail shaft housing — eliminating the erector set shift linkage used on earlier units with shorter tail shafts. This is a link to the Magnum XL web page =>
Magnum XL
This is an image off the Tremec site, of a Magnum XL;
Notice the extended tail shaft and rear shifter mount. Unlike the Magnum version with a TKO rotatable shifter providing two different shift locations this unit has a single rear shifter location and a moved rearward tail shaft mount. It also has the same mid-shift location as the regular Magnum.
There are specialty gearsets that are available for the Magnum series of transmissions that can produce a very close ratio set of transitions from gear to gear. The ratios are 2.29 first, 1.60 second, 1.21 third, 1.00 fourth, 0.84 fifth and 0.67 fifth.
Along with improved gear size and strength the Magnum series transmissions also produce an improved shifting experience with double and triple cone synchronizers that make the transmission very easy to shift even at higher rpms.
Ed
p.s. Almost forgot the TR-6060 is the OEM version of the Magnum