It may have changed by now, but I know when they first started using the BTR center gear sets were hard to find, and only available in a handful of ratios. I know there was talk at one point about it being essentially the same unit as a Pontiac GTO so their gear sets might fit? I never heard one way or the other if that was correct or not.
I find I'm happy with the 3.46 and the TKO600 and .82 OD. First gear is up in smoke before you say boo on my Roush427R motor, your FE should be more than enough for your tires
I do wish I had a little more OD as I find I cruise 65-70MPH at 2800 or so. using 295/50-15s by the way. Most of the calculators say I should be turning fewer RPMs but I've checked the speed with GPS and a chase car and it all seems to check out.
Daryl