My 1991 d-350 service truck weighed over 9,000 lbs loaded with its service bed and tools. I pulled a trailer with it several times a week, it never complained and had plenty of power.
I did install an innercooler from a '93 along with injector timing, and fuel adjustment. It was not enough to make it a hot rod or anything. I sold it with over 280,000 miles on the odometer and it is still going today with similar duties and weights.
The rotary pumps are much cheaper to rebuild than the P pumps, but not many people consider that. I like them personally, very reliable if you don't crank them way up.
Your excursion should be a nice rig with the 5. 9. It is my ideal family car idea. I'm looking for a v-10 excursion to repower, but it will be a while before I have the time to do it. The v-10's are getting put on the back of the dealer lots down here, most are growing cobwebs and looking really promising as a repower project. ($10,000 or less is what I'm looking for)
Only thing to consider is emmissions, if you are subject to inspections or any of the other regs, you may have a problem using an older engine in a newer vehicle. Most engine conversions that are compliant with the regulations have to use the same year or newer engine. Tailpipe tests are coming for the entire US, just a question of when
I'd rather see a vehicle with a bad engine put back into service than be junked as it is just as viable an arguement to keep the car MAINTAINED (not just driving it down the road blowing oil/antifreeze clouds behind it), and save it from the scrap pile. Which is only contributing to the waste disposal issues in this country. It seems just as green compared to the industrial waste that is part of every new car built to replace it.
Attention: TDR Forum Junkies 
