On our farm, for hauling forages (and grain), we have 4 home made haul trucks/dump trucks:
F750 - 16ft x 6ft x 8ft box w/6.5L Diesel (~140-160hp), 6 speed manual. This truck is old reliable. It doesn't do anything fast, but it always starts, always makes the turn and runs all day just sipping fuel, back and forth from field to farm. It'll run 55mph, wide open empty, and maybe 45-50 with 8-10 ton of silage in it. Also gets used as our own portable dumpster for hauling construction waste to the local dump once or twice a year.
(2) L8000's w/ 18ft x 6ft x 8ft box w/7.8L Diesel (~200-250hp), Allison Automatics. Both of these trucks are tandem axle's, one has a double frame for hauling grain, the other is just a single frame and only a forage hauler. Both trucks are quick, plenty powerful and have no issues running highway speeds, plus anyone can jump in and drive them because of the automatic trans.
L9000 w/22ft by 6 ft x 8ft box w/6v92 Detroit Diesel (350hp), Eaton 9-Speed, Tandem axle w/tag axle. This is a serious truck that requires an experienced truck driver to operate. It's long, has a horrible turning radius and the DD engine/Eaton 9 speed is not an easy combination to master. It does, however, allow us to haul 750-800 bushel of corn/beans/wheat to market at normal highway speeds. 350 hp makes this truck get up and boogy down the road, even with a full load on.
The F750 and both of the L8000's started out as delivery box trucks that we converted to haul trucks. The F750 is still a single axle truck, and we put tandem axle's under both L8000's to increase payload capacity. The L9000 was a straight body, milk hauler that we converted to our large grain truck.
I just wanted to share what some of the pros/cons were of the models I'm familiar with. The 6.5L and the 7.8L Ford Diesel's are amazingly reliable engines that start very well all year long and haven't given us any issues. The Allison Automatics have been just as reliable for us, even though they were old to begin with and are run in dry, dusty fields all year long. The 6 speed in the old F-750 has been kind of PITA the last few years as it has a lot of wear now and needs to be shifted very precisely or else the shift mechanism comes apart. We rebuilt it, but only parts available for this trans are used parts so it'll only last so long. We're kicking around the option of converting that truck to an Allison as well, though we'll have to change the Ring/Pinion gears in the read end if/when we go that route.
Converting a truck can be done and for quite the savings over buying one pre-built, but there are a lot of small things that come up when designing the hoist system, hydraulics system, re-routing exhaust, etc that rack the cost up quickly. We've also had our issues along the way, but most of those come from running loaded trucks through rough fields (broken springs, snagged lines, etc). We're also currently overhauling the 6V92 in the L9000 due to a spun main bearing ($$$).
