Any and all the mods you could want, although the automatic doesn't react well over about 45 horse in mods. You'll want to replace the lift pump (know problem). They fail and burn up your injection pump ($1200 part). You'll also want to check for what is known as the "53 block" Blocks with 53 cast into them are more prone to crack. Number should be cast on the lower driverside of the block. Some people are having issues with the dash cracking and some are having problems with a vibration at freeway speeds believed to be the d-line.
I have 145,000 miles and I cracked the dash. Still original pump but I want a fass. I havent had any leaks or doesnt use any oil, at all. The only problem I have had is the auto likes to stick during winter cause its cold and for some reason we had Ujoints on driveshaft go out. But its all find now and hopely get tranny rebuilt/beefd up before long as well
All in all even with the minor problems associated with that year ( and what year DOESN'T have it's issues ) none of them would scare me away from the truck at all. Every issue ( except for the 53 block ) can EASILY be addressed.