Been there.
I first operated my '77XT with a T3/T4b (.63 SII turbine, 50 trim comp, internal wastegate) with a 3:90 rear. The car had a good feel to it, spoolup was super fast, but it didn't have the power I expected or achieve what I thought was enough boost for that setup.Boost control with my HKS EVC was effective and crisp.
I then switched to a 3:36 I had laying around the shop and the car was transformed - boost in abundance! Problem was, now I had too much, and my smallish internal wastegate wouldn't keep boost from spiking. Spiking isn't the correct term, because boost would go to the desired setting,linger for a brief time, then climb right up uncontrolled.
I had decided to go to a big external wastegate but before that happened stupidly kept tinkering with the car. One day while trying some hairbrained scheme (that I won't elaborate on because it's embarrassing to talk about)I spiked the boost in major fashion and after that the turbo was noisy and broke - (compressor surge?)
I would have liked to see how the original turbo worked with the 3:36s and big Turbonetics Racegate that I installed next, but at this time I had Reed at Turbo Specialties build me what he calls a T61 - Essentially a T4 in a T3's body.
After some (lengthy and tardy) warranty fixes to the turbo, this new setup is a monster.
The big turbo has 12 psi by 3200 rpm in any gear,and boost control is rock steady, absolutely straight line at any boost level from 7 psi on up to about 18 psi. And there's lots more there when tuning and race gas permit.
This has gotten a bit long -winded, but the point is, Turbo cars like the load characteristics provided by long gears. ( Can I have an Amen Scottie ) The bigger the turbo the longer the gear you need.
I think I'm going to try to talk Ross C. out of those 3:15s he said he has.
Also, I feel your pain, brother. Throughout this deal I've become a diff-swapping expert.And I've just pulled the diff again to have some LSD problems worked. Isn't wrenching fun?