Diesel Bombers - New guy here, just got a 1990 d-250

First time posting here. After selling my '03 cummins due to the price of parts, I decided to get into another one, but one that I could afford to own. After looking for about 6 mos, I finally picked up an automatic 4x4 d250 with an 11' flatbed for 5k. Not cheap, but cummins are like the dodo bird these days, good luck finding one.

I used to pull 15k with my '03 automatic, I don't expect that out of this one currently, but I intend to change that. I was wondering if it's worth building the auto or just converting to manual. Also wondering if anyone knows about intercooling one of these beasts. I'm going to put in a new steering shaft from jegs, there appears to already be an aftermarket turbo installed as well. I've also heard rumors of a killer dowel pin I intend to investigate.

So to the guys who tow the heavy loads, what should I do to this thing to make it reliable and bad ass at the same time. Planning to work on it throughout the winter. Thanks for any advice you can give, I want to put my $ in the right direction.