Time to upgrade
A few years ago I blew an F-600 motor right after I finished fixing the truck - I was about 10-12k into the truck at this point, and it seemed to be great. The options were used motors - I could NOT find any for the two months I looked, new motors - from Ford, with no guarantee on the core money being returned because there was no way to know what the bad block looked like, and this was going to cost about 4000 to complete correctly. I had the 370 gas with an Allison Automatic. Switching to diesel was way too costly for a 1984 truck. I cut my losses, sold the truck for 1500 and picked up a 97 F-800 with a cummins 5.9l diesel and air brakes, 24,500 GVW. This worked out great even though I have a payment. I would advise against working too hard to fix the truck you currently own. I see the truck all the time, since I sold it to another landscaper who buys stuff at our nursery. But, he put a 460 into it, and it turned out to be even more than a pig than before (he complains about it eating fuel all of the time, and I can tell it seems to have less power than before). I think the biggest deal if you do end up changing blocks is the connection between the tranny and the motor. And you will probably have an easier time with the spicer than I was finding with the Allison.
As recently as last week, a friend of mine picked up a used (1995) F-800 with a cummins diesel. It was traded in to the dealer as a cabinet delivery truck with a 24' box. For 20,000 out the door, my friend had the dealer cut down the frame, and a 12' dump body was installed. This is something you should look into before you waste time and money fixing an old one.