I ran 35/12.5/15 with full crank and minor trimming to the bumper no rub on my 88. It had OK power. Would I do it again? Hell no. I'd run 285/75/16 with no crank or minor cranking. BTW I also have 3.42
I personally know of a 350 TBI that with road em for days and still pull 16 mpg with a 5 speed and 4.56s I can only imagine a vortec. I'd say open the trans up and check all the gears this weekend too if you can.
You could try to ring a 14 bolt sf out of a 454ss and have 4.10s but that would be like finding a needle in a haystack. You'd could also have a 6 lug 14 bolt sf redrilled to 5 lug or even 6 lug swap. Otherwise you're stuck with a 10 bolt