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check out www.ford-trucks.com they have a forum section just for Ford diesel vans. I just read a thread on a van specific exaust and a PSD van fuel system mod. Also has info an chips and propane.
I can't recommend any PSD upgrades, since I have an IDI.
As far as the tire suggestions, that would depend on what you use your van for. My van is a daily driver that sees alot of muddy job sites and muddy trails to the summer fishing holes.
Right now I run 35x12.5/ 16.5 BFG MT on 16.5x8 wheels (had to do some minor trimming on the front fender and bumper). Planning on going back to 245/75/16 on factory alcoa. Looking a Interco Trux MT tires ,if I can get a reasonable price here in Alaska. They claim to be quieter than BFG, plus they are siped in the center lugs for better ice traction, and also come in E-load.