My K5 has a class III weight-distro hitch, the factory Z82 tow package, tranny (temp and pressure) gauges, notably more power than the original 305 wheezer, and a beefed transmission. It tows something like a pair of trailered horses or a '63 Ford Galaxie 500XL ragtop on a trailer without complaining or acting squirrelly (front sway bar, good shocks, stabilizer).
However, I don't tow on a regular basis. I see solid 70s and 80s extended cab longbed 3/4 and 1 ton 4x2 and 4x4 diesels in the local classifieds every day of the week for pretty good prices, and I'd buy one of those if I was planning on towing heavy all the time. Here are a few examples:
'86 FORD F-350 flatbed, dually, toolboxes, ladder rack, 6.9 diesel, $2750/OBO.
'87 FORD 3/4 4x4 diesel, auto, ext. cab, longbox, runs good, $2500.
'92 FORD F-250 ext. cab, 7.3L diesel, 5 spd., longbox, $2300
Looks like it's a Ford day in the local rag! The flatbed dually sounds interesting. They might need a few rough edges hammered down, but any local paper should have several ads of a similar nature.