Obviously the only way to truly know what is inside is to drop the cover and count teeth. But I would trust the codes unless something did not add up. Like a supposed 3:08 rear that made the engine run at 5000rpm in high gear. The high gear ratios I posted would be used with an overdrive transmission with lock-up converter. OP: what tranny is in it? The Tahoe would be a 4L60E four speed with overdrive.
As for the great body work vs the shadetree mechanicals, I have never seen a body man who was a good mechanic and vice versa. They are two different skills and it would be a unique person that could do both well.
That said, I know many Spanish guys that do incredible body work and painting. They have a really good eye for that. When I was in Ft. Bliss many GI's drove across the border into Juarez for some paint work. One of my buddies from Detroit had a 1970 Bonneville. They not only made it look brand new, they did the interior in leather. Very inexpensive too.