The T5 isn't as closely spaced as a T10. The spacing on a T10 is often considered ideal.
What you get with a T5 is a better first gear ratio and an overdrive 5th. I didn't like the gear spacing and went with a Richmond (Doug Nash) 5 speed.
With that transmission first is 3.26 and 5th is 1:1. Gear spacing is modeled after the T10.
What you do to get the effect of an overdrive in 5th is by dropping the rear end final drive. Depending on exactly how hard you want to accelerate out of first you either go to 3.00, 3.25, or 3.50 rear ratios. Of course you could go to a 2.50 or 2.75 rear if you want to cruise like a Lincoln in 5th.
I have 3.50 s in right now. I have a 3.25 ready to go in. With 3.50s you can almost pull the front tires off of the ground by letting out the clutch normally and nailing the throttle. Tire dealers will love you. You will be in there often for replacements.
Generally speaking, production T5s are not very heavy duty transmissions. They are a bit on the light duty side. You will be ok with a 289 but the more cubes that you load on it, the sooner you will break it.
Nothing is foolproof though. The FIRST time I drove the car with the Nash, I twisted the shift cams off. Seems they were two part and "friction welded" together. No one told me until after I broke them. Then EVERYONE SAID, "sure, you didn't know?" Some guys can break anything. I get at least an honorable mention on that list.
You need to use a 1/4" spacer on the original bell with the T5 and you need to finagle the shifter. Other then that it's a bolt on.
The Richmond I used was the Ford 24" length with 31 splines so the driveshaft remained stock.
Depending on which T5 you use you may need to mess with the stock drive shaft.
Incidentally, the Richmond is nice in a GT500 as well.