First, should I go with a new truck, or a good used truck? If you work several states, and run a lot of miles between jobs, a new truck might be justified. But, if you mostly work within 50 or so miles, a good used truck might be a better investment. How many miles per year do you drive? Since most of us are probably going to end up re-mounting on a used truck, I'll limit this discussion to them.
Next, a serious consideration should be, gas or diesel. They both have their pros and cons. A gasoline unit will probably be cheaper, and easier to repair, to start out with, but it will probably need more maintenance and use more fuel. Also some gas-powered trucks are quite a bit lighter than their diesel counterparts, and may be too light duty. A diesel truck of the same size is usually heavier, and is probably heavier built, too. Larger axles, springs, frame, brakes, that kind of thing. They are generally longer lasting, but more expensive to work on. Fuel mileage might be quite a bit better.
One thing to watch if you are converting a gas-powered rig to diesel: Diesel engines usually make a lot more torque for a given horsepower rating. Torque is what you need to pull out of a bad spot, but you can overdo it. Check the torque rating of the power divider between the truck and the rig. It may not be heavy enough. If you don't have the book, look at the through-shaft on the divider. If it is a lot smaller than your driveline, you may have problems. Is the wheel base the same as the truck you have now? It's been my luck over the years to NEVER find the right length truck when I get ready to re-mount a rig.
Seems like rigs all come from the factory on a truck that's a different length than anything in the known universe. I've shortened and lengthened trucks to fit rigs and I can tell you for sure shortening is easier. I know, the factory guys tell you not to cut or weld on a frame, but it can be done. If it's a double-frame truck, stagger the cuts, and fishplate the welds. If it's a single-frame, go to a salvage yard and buy a piece of frame from a double-frame truck of the same type. Scab it into the inside of your new frame, overlapping the welds.
One note on frame modifications. It will make them somewhat weaker. Usually this is not a problem, except in the most severe duty.
There is one type of rig on which I would not consider cutting the frame. A rig that has the front leveling jacks on the front bumper stands a good chance of cracking even a factory frame, let alone a cut-and-welded one.
A better design is to have front jacks under the front of the rig, rather than the front of the truck. This minimizes frame stress during drilling.
Re-mount time is also a good time to look at some of the smaller things, you've been wishing for over the years. Tired of those old mechanical jacks? This is the perfect time to go to hydraulic jacks, and maybe a bigger hydraulic system. How about an auxiliary winch, or a winch on the front to pull you out of that mudhole. This is the time to rig it up.
Remounting isn't usually too difficult if you are going to a truck of the same size. The rig frame usually lines up pretty well with the truck frame, and the only custom parts will probably be the drive shaft(s). Tail lights usually need to be moved, don't forget to 'armor' them or they'll be knocked off by the third job.
A 'truck transplant' isn't as big a job as you might think, and you sure can breathe new life into an old rig with a little work.
For comments, or questions, you can e-mail me at ROCKBIT@compuserve.com.