there is only one brand of T56
http://www.ttcautomotive.com/ The T stands for Tremec (formerly BWA) which is now Transmission Technologies Corp. there are many VARs (Value Added Resellers) that offer installation kits that includes brackets, clutches, etc., however the transmission is still manufactured by TTC. TTC then sells the transmissions to the OEMs and aftermarket
the question on electric (VSS) vs mechanical on the speedometer output will be answered by a few questions:
New trans or salvage yard find?
stock gauges or electric speedomter?
Carb or EFI requiring a VSS?
if you are buying a new trans, then you can purchase the aftermarket/retro - custom version of T-56 originally marketed only thru Sallee Chevrolet for use in 3rd generation F-bodies and street rods. this has the mechanical speedometer output version with slight changes in the case.
http://www.sallee-chevrolet.com/BorgWarner/bwt56.cfm , otherwise the standard offering and all factory installed T56's have a VSS installed. if your running factory gauges, then a mechanical speedometer output is suggested. if your running aftermarket gauges, most are now electronic requiring a VSS. and if you are running EFI, then you require a VSS
now on to mounting. as Dave mentions, you need to mount it properly. the T56 is too long and heavy to cantilever if you are mounting behind a 235 with an adapter. doing so will put too much of a moment strain on the trans to bell housing joint and parts will break. if you are mounting it behind a V8, then you will need the trans mount and side engine mounts (common 3-point isolation mount)
The T56 is larger than the T5 or original 3 or 4 speed. so a bit of interference may be encountered. I know when installing a T56 into a few projects the transmission tunnel needed to be "adjusted" with a variety of hammers and some lines and wiring needed to be re-routed.
If you are installing a salvage yard find, have the 3/4 shift fork updated when the trans is out, and replace all the nylon sliders. that is a known failure point on the early transmissions.
