3.73's in Mustangs behind T5's is a very common option so you'd be OK. You would see a 15% RPM increase at given indicated speeds. So if you are turning 2000 RPM at 60, and went to the 3.55 you'd see 2300 revs which isn't that bad. And it would be a VERY noticable off-the-line feel improvement!
The T45 is for the modular motors (4.6, 5.4 etc...) and has the long input shaft and the mod bellhousing so write that one off. If it were me, i'd go to 3.73's. Just do the math and see where you are comfortable. A 3.73 would give you a 21% RPM increase (3.73/3.08=1.21); so back to 2000 @ 60 would become 2420 @ 60 with 3.73's. And with a .63 overdrive, you'd still have great cruising with a functional 2.35 rear gear! One thing all this DOES assume is that you have a V8 T5. Some of the oddball T5's had VERY low 1st gears, but these were mostly 4 cylinder transmissions. After you change gears, just be careful the first time you put your foot into it......
