I can recall - from back in my XR4 driving days - going over online info regarding swapping in a T5 to replace the T9, as the T9 was thought to be torque-limited when beefing up the 2.3 output.
I think the mustang 2 parts are one of the better choices (along with Miata front end parts) for the locost front end, simply because they're so well supported in the aftermarket. Speedway motors sells all sorts of new MustII parts, including spindles with a 2 inch drop, brake kits, new manual and power racks, etc...
I've already got a set of Miata spindles stashed away for the next proj. but if I didn't...Mustang II would be plan B, y'know?
as far as making up a hybrid driveshaft from T5 and T9 parts - and coming from my experience succesfully putting together the B-3 driveshaft - I would likely have ago at simply cutting and mating the two somewhere along the length.
I do recall my 89 XR4Ti had a rubber Isolator (the infamous Guibo) just after the tranny output U-joint, then a length of shaft to the center bearing, and then the secondary shaft to the diff housing. IIRC, the T5 driveshaft is one piece from the tranny tailshaft, to the rear axle (Live axle on the Tbird Turbo coupe)
If it was me - and I know it isn't
- I would cut the T5 driveshaft somewhere along the length from the tranny talishaft, and then cut the T9 driveshaft similarly, and weld the two together. I'd fixture it for straightness and concentricity, and make sure the weld was solid, but otherwise, would have a go. Too Busy welded up his own hybrid driveshaft for his Centaur rebuild.
what are the diameters of the two driveshafts?