I had a 302 AOD in my 89 when I got it... I then put 4.56s and 35s on it and it was GUTLESS still.... If you are planning on going to 38s, I would say that 4.88s ( is that not the lowest gears you can slam in stock diffs?) and expect to need the power of a 351W to spin them up in the mud...
If you plan on leaving the AOD ( if thats what you have ) behind the 351W and 38" tires behind it , expect it to blow up too. And if you plan on running 38s on 1/2 ton diffs carry spare axles....
Bob
Bob