You have done mopst of teh work needed to keep the truck running for years to come, and you wouldn';t have had to do so much if teh PO had done regular service on it. You would get more power out of teh engine if you installed 4.56 gears, the 31's are 11% bigger than the 28's for which the 4.10 gears were designed for, so at 65 you are really doing 70+.

Granted your Toy isn't going to ride or sound like a truck 25-20 yrs newer, and if you plan/desire to tow something large, than you should really look into a full size rig.

If you don't need the extra money from the sale of the '85 to finance a new rig, I would keep the '85 until I was sure I didn't want/need it anymore. It is paid for and with a new engine it is going to last a lot longher than any domestic full size will especially if it is going to be used as a daily driver.