Yeah, but there's nothing sacred about that angle. If the engine tilts back another 1°, so what. Match that new angle with a similar adjustment at the rear, and you're golden.
I remember my AutoTech Instructor telling us that the slip yoke to drive shaft angle and the pinion to drive shaft angle should not differ more than 4-5* else you get drive train vibrations. There were alot of Off road guys in my classes so he would constantly receive questions regarding lifts, and he would stress their negative effects on drive shaft angles.
I never did any angle adjustments when I put my 351c with the T5 in my HT. I Just used the old style cast motor mounts, and a swap trani mount and cross member, also used a jmc hydro clutch kit( aren't offered any more). Never got any vibrations all the way to 5k, never went pass that because I doubt the 8" I have in that car would hold. I doubt drive shaft angles changed much throughout the earlly mustang years don't really know, but eh, if it ain't broke, don't fix, right?
Id recommend a WC T5 out of a 89 and newer 5.0 mustang, That's what I have, they're good to 300 lb/ft torque from what I've read. I also used the Fox body bellhousing, but I wouldn't recommend this if you plan on putting out some good power, I noticed I was getting some small stress cracks forming near the reinforcement ribs.
As far as fly wheels goes, you'll need a 157 tooth 28oz imbalance for the early sbf engines. and a 157 tooth 50oz flywheel for the later 5.0l engines. I believe the 5.8l cobras of the 90's also used the 28oz flywheels. And yes you can use your old fly wheel as long as you have the right balance and its a 157 tooth, so you can run the fox bell housing and the smaller fox high torque starter, if you want. If not they do offer the spacer kits for early bell housing to T5.
Id recommend the newer style clutch bolt pattern, so you can have a wider selection of clutches, as well as being able to use the factory ford diaphragm pressure plates, not the 3 pronged pressure plates the older ones did.
Hope this helps a bit.