Try using a micrometer on the input shaft.
6 & 8 cylinder T5 - pilot shaft is .668"
4 cylinder T5 - pilot shaft is .59
The 6 & 8 cylinder T5 use a different pilot bearing from the 4 cylinder T5.
The 246 T5 is from a 94-95 Mustang and the input shaft is longer. To use it, you would need a bellhousing from a 94-95 5.0 Mustang
T5 identifier information.
On the tail shaft of the transmission, there will be a stamped aluminum tag. The tag will have a number on it that you can cross reference to the chart below. There is no other way to find out what type of T5 it is without disassembling it. All the rest of the numbers on the outside of the T5 case are part numbers which are not unique to any particular T5 model.
Remember that 94 - 97 T5's have a longer input shaft (about 11/16” longer) and can only be used with a 94-95 bell housing unless you modify or replace the input shaft.
T5's used with a 4 cylinder have a 3.93 first gear, reduced torque ratings and an input shaft pilot diameter that is smaller that the T5 used for V6 & V8 engines. The pilot diameter is .59" compared to the .668" used on V6 & V8 T5's. It requires a different pilot bearing to be used with a V6 or V8. The pilot bearing you need is for a Ford Ranger diesel from AutoZone part #14672
I don't know how you feel about the DYI thing when it comes to gears, some people don't like the idea of working on transmissions. If the DYI transmission fix idea isn't something that you would rather leave to someone else, here are some resources:
See
http://www.ttcautomotive.com/English/onlineorder/product.asp to download a FREE service manual for T5 or Tremec 3550/TKO
You will need the Adobe Acrobat viewer which is also a free download –
http://get.adobe.com/reader/
See
http://www.hanlonmotorsports.com/ or
http://www.ddperformance.com for parts
