If you go with a T5, then look up the Ford Z-Spec version (still available brand new for about $2000 - checkout the following link below). But, you will have a very big RPM drop between the 1/1 4th and the .63/1 overdrive (which I do NOT like). What can be done to make the OD a lot better is switch out the OD gears to what was once known as the S80 gear set (was used in road racing). You can now buy the .80 overdrive gear sets for about $250. I just rebuilt one of these transmissions last Saturday and installed the .80 overdrive 5th gear - is for a road-racing Mustang that we have fun with.
Link to the Sebring Gears and Mods:
http://www.moderndriveline.com/Techn.../80gearset.htm
Also, note that there is quite a bit of work to get this whole transmission to fit in your frame (modifications required), make the necessary modifications for the new shifter location (floor pan), make extensive mods to support a torque-tube setup (special transmission case and machine work) or switch to an open drive. What I will say is that you'll surely love it once you have it!
One last thing to mention is what brakes do you have on the car? If you want to go 70 mph with mechanical brakes and actually stop the car in a reasonable manner, then you are in for some trouble. I find that properly setup mechanical brakes do pretty well up to about 55 or so mph.
The faster you push these cars with mechanical brakes, the more dangerous they become. This is why I'm switching over my 32 Cabriolet to Lincoln style hydraulic brakes - when I put the Columbia in it . . . I need to be able to stop the darn thing!
New T5:
https://www.summitracing.com/parts/f...03-z/overview/