The transmission in my 55 2nd series has been changed to a later model. The trans has the following numbers cast into the side:

GM23
8825647
F82

The following number is stamped on a machined spot above these on the right side of the transmission:

12w419838

Can anyone tell me from these numbers what transmission this is?
Since it is not the original transmission, is it the same or a different length as the stock? Length measures 21.5 inches from front to back. How will this effect a change to a T5? Will the T5 still be longer than 21.5? I am trying to figure out if I will need a shorter driveshaft.

Thanks,
Hugh