I have a 700r4, BB & 3:42 rear.
For the tranny the crossmember had to be moved back, the driveshaft
shortened, new fluid lines fabricated, installed external cooler &
highly recommend you do the same & my console shifter works w/out
mods.
At 70 I run about 2100 rpm. My mileage has not stabilized yet, still
breaking in. This tank looks to be about 11mpg, but I loose
control of my right foot every time I am in the Monte
Go here and peruse some. They are pretty helpful
if you call also.