Looking at the gearing ratios, the 86-93 mustang T5 transmission really does look like it has the ideal gears for this setup, providing slightly lower ratios than stock for the starting / 1st gear. I don't find myself wishing 1st gear had more power in my 1985 280sl with 13.68, rather wishing I didn't have to switch out if first so quickly with my 3.58 rear end. It would be far worse with a 4.08 rear end. It looks like the MB transmissions I have and the getrag 265/6 I have all have higher revving gears, especially in 1st, 3.82-3.98. The 3.35 first gear of a T5 3.35 first gear combined with the 4.08 rear end is pretty damn perfect at 13.67.
^^^ my 1985 280sl
^^ table with mustang T-5 gearing.
0.68x4.08 = 2.77 for overdrive is pretty damn nice too. You might find yourself downshifting into fourth for any passing, but it will be nice to know you will have that low gear overdrive when you want it.
I was thinking: "let's just bolt up the 4-speed" or "let's try the getrag 265/6, but after reviewing the gears, I'm totally on board with the T-5. Otherwise, I think you are going to be searching for a lower differential ratio somehow, and other than getting another rear end from a 280se 4.5 (3.27?) I'm not sure what you could do. I guess that IS an option and you could use a getrag 265/6.
So... Next step on the drive train seems to be either:
- find a T5 transmission from a factory mustang 86-95, or
- find a 280se 4.5 rear end.
For the steering box, there's a 280se 4.5 parts car not too far, but unfortunately the rear axle is gone already.