Here is an adapter I make to put a jeep t5 behind your flathead and keep your torque tube. The one thing you do have to do is drill and tap the end of the jeep t5 output shaft. I have a few made up if anyone is interested. PM me for more details.
Yes.
How much longer than the traditional Ford 3 speed is the T5 with the adaptor? (How much does the torque tube / drive shaft have to be shortened?)
which transmision do i exactly need for this conversion ?
I agree with Coop on the Chevy case except it is possible to use a Ford pattern T5 adapter with a hogshead (my preference) and the Ford mustang T5 tend to have the better gear ratios from the get-go. If you are wanting to use the alum bellhousing then the Chevy pattern T5 is the way to go.
Both the Ford and Chevy input shafts come in different lengths, but it might be a little less common among the Chevy line. I was not familiar with the jeep being NWC only. That would make the Chevy a better option (maybe) as the NWC Fords tend to have the long input shafts.
Coop, that is a clean setup!
You guys ought to put together a "T5 Bible" if it hasn't already been done.