[skyking]

'96 cummins Dodge here.
I've done many of the mods listed above, except the coolant change. I installed a big block style 190 thermostat, I can get a NAPA part number or you with a little searching.
The Cummins t-stats are expensive and prone to rolling the rubber gasket into the works, causing them to stick.
No need for a fuel pump change, a fuel pressure gauge will tell you if you need it.
Stock air cleaner will support 350 HP if you get another hole in the airbox, the fender holes is just not enough.
Of course supporting 350 HP is not getting good fuel economy either

I found that after I added power, I have a hard time not using it. That "adjust your driving habits" part is a big part.
I have gotten back to the best towing mileage at 14.9, but I have not broken into the 20's yet unladen.
What kind of MPG's is he getting now?