I'm not sure I'd jump to the conclusion that your 3.08 gears won't work out okay. I have 3.08 gears in my '55 1st Suburban and while it doesn't pull a loaded trailer very well, running around empty its fine.
I have an SM420, not a T-5, but if you don't pull a trailer and only use 5th on the highway I suspect you'll do fine.
If you do pull a trailer the T-5 is the wrong transmission anyway.
One of my other vehicles has 3.08 gears and an OD that drops the ratio to about 2 to 1. That OD gear isn't much good around town, but on the highway I'm loafing along getting 26 MPG from a 3000+ pound car that smokes the tires at will. Of course it won't pull a trailer either...
