There is absolutely no internal difference between a T-5 out of a 4cyl and a v8, the only difference is the input shaft has less splines on it, and as long as you specify the number of splines, any parts store can get a 5.0 clutch to match it. Trust me we have swapped numerous T-5's out of 4 cyls into V8's at my shop!
It is a common misconception that the 4cyl and V8 T-5 tranny have some sort of internal difference. If this were true than ford would never have been able to put out as many as they have and they would have been outrageously expensive.
The only difference in the V8 and 4Cyl drivetrain w/ the exception of the motor is the rearend!
*Edit*: Well, time to admit my mistake...I just talked to my mechanic in the shop and he said there is a gearing difference, but he said they are interchangeable. Damn, I hate admitting when I am wrong. My old mechanic said there is no difference at all and my new one is the best with standards so I tend to believe him.