Rear diff gears
 

So I’m running 20x12s on my 88 with a 318 and I want to know what would be the best rear differential gears because I want to use 4.10 but I’m afraid with me going 75 mph my rpms are going to be skyrocketing. I want to have enough torque to where I can haul trailers too without blowing my engine up

What size tires?

33x12.50/20

4.10's would be great with 33's. :) You may run a little bit higher rpm on the freeway, but, for the most part, it'll be much better than the 3.55's.

I appreciate that very much man