I would make the decision on which transmission you plan on ending up with before i would select a rear end with emphasis on the gear ratio.
A SM420 will need highway gears in the rear end. A T-5 will benefit greatly from a rearend ratio between 3.73 and 4.64, depending on the the overdrive ratio that comes in the T-5.
If the 5th gear ratio .86, a 3.73 rearend would likely be optimal.
It the 5th gear ratio .73, a ratio as high as 4.64 would be recommendable.
Keep in mind that you can dial in the setup by selecting a specific tire diameter.
I have a T-5 with a 2001 Nissan Xterra six lug (Chevy pattern) limited slip axle with a ratio of 4.64. I tweaked it into perfection by changing to 30.5" diameter tires.
With this set up, I can pull a tree stump, and drag it down the road at 80 MPH.
