It is definately worth the partial trade, you could easily get $1500-2000 if it is running and onle cosmetically beat up, a clan one can bring a price of $5000.

Since you have already started modding the '85, I wouldn't swap the parts over, it would be alotr of work to get the '92 to the same point the '85 is at now. Also the 3.0 has a HG problem, can be fixed using the right parts, i.e. making sure the heads and block are flat, swap to headers, and us high quality metal gaskets, and swapping from head bolts to studs. Even if you did this, the 4-spd auto sucks most of the power that the 3.0 has, also adding dual t-cases runs about $1000 more for the 3.0 than the 22r's

If it runs and only needs the fenders replaced, I would replace them and resale the truck. You can get new unpainted fenders for about $45-100 each, you could also probably find used color matching fenders for about the same or less. Check out craigslist.com for your area/region or local wrecking yards, pirate4x4, Marlincrawler for someone parting out a rig.