i have a question about a frame on a 99 dodge ram. has anyone ever changed the rear frame on a 2nd gen dodge? it looks fairly simple but is it?thanks

Wondering the same thing about my 99 Dakota

I'm going to assume both of you are considering cutting off the original frame and replacing it with another rear section of the same year.
I have not specifically done either truck, but I sure have shortened several truck frames, so I believe that probably qualifies me to speak.
Choosing a place to make the splice is very important. You need a straight and level section at least 12" long. The best location is an area where you have a "C" channel section and where you have access to both the inside and the outside of the frame.
The next most important thing is to be sure you can check the straightness of the project throughout the entire process. The finished product needs to be level, square, and straight. That may require removing a lot of bolt on accessories like the bed, fuel tank, fuel & brake lines, emergency brake cables, and possibly even the rear axle and springs.
The entire truck needs to be off the ground and levelled on jackstands, supporting both the front and the rear frame, and the frame on both sides of the splice location. Nothing can move when its cut apart, this may require more then 8 jackstands.

Contrary to popular belief, when the frames are extended on big trucks (semi tractor, dump trucks, fire trucks) there is no welding done at the splices, and has not been since the mid 1990s. When the frames are cut (at a straight 90 degree angle), there is a piece of channel iron formed the fits tightly inside the original frame rail for each frame rail on the truck. The new rear section is butted up tight against the cut on the front section, and the formed channel is clamped into place and (4) 3/4" diameter holes are drilled in each the front section and the rear section (8 holes total, on each frame rail) and the inside channel is bolted to the inside of the original frame with (8) grade 8 bolts on each frame rail. Before the bolts are torqued, the square, level and straightness of the frame is checked, and then it is checked again after both sides are done. I believe on large trucks this needs to be certified.

On smaller trucks, the splice can be welded by a qualified welder, but the same care to be sure the frame is level, square, and straight is still required. Most light trucks forgo the formed inside channel, but adding a diamond shape fish plate on both the inside and the outside of the rail is a good plan. I've seen guys step the cut rather then a straight cut on the frame rail, and I've seen guys that insist on making the frame cut at an angle. I believe as long as the splice has been properly welded and fish plated, I believe a cut at other then a 90 degree straight cut probably only makes it more difficult to be sure everything is true. I've also seen guys box in the splice area, which doesn't hurt, but probably not required.

Having access to both sides of the splice is important, and getting everything straight again is the most important, followed by good welds. Gene

I don't know where you got your info but I have to respectfully disagree that this is a normal procedure for cutting or splicing a big truck frame.

The procedure that was done on the last one I had stretched was to cut the frame at a 45 on one side and at a 45 in the opposite direction on the other side and weld it. Suspension and crossmembers were moved back the 16" I increased the wheelbase by and no fish plates. The job came with a pile of documentation for how and where it was cut, the welding procedure and a brake application timing analysis to make sure the extra 16" of airlines didn't take the brake application time out of spec.

I agree completely with the rest of your analysis. If you aren't intimately familiar with precision measuring, a square, a level and a welder, it's best to leave it to a professional.

Kevin

I worked at Monroe Truck Equipment for a few years while they were building over 100 trucks for one of the train railway companies. Every truck had a crew cab conversion and a frame extension. All of those trucks were done with the process I outlined, They also built tandem dump trucks with center blade snowplows for many states, as well as several fire trucks. The outline I wrote was their standard procedure but that was several years ago. Things may be different these days, again. Gene

Well I believe the 99 full size trucks are built like my 99 Dakota frame is meaning that the front and rear frame sections are riveted together just ahead of the rear axle hump, as well as the front spring hanger is riveted thru the overlap section. Mine isn't as bad yet as some I have seen but the rear frame section likes to rot right behind where it overlaps the front section. What I would do would be to cut the donor rear frame section just ahead of the overlap and grind/cut/drill out the rivets holding the 2 sections together, inspect and coat the parts of the frame that butt together to form the overlap and bolt it back together.
I think that the op is talking about something similar.
Whether STD cab, club cab or short/long bed later in the truck's series I believe that they all used the same "butt" portion of the frame, with only the front portion changing between trucks depending on cab/ bed length combo.
(And yes before anyone asks yes I know that the Dakota and the full size frame won't interchange but by this time they were building both series of trucks in the same manner)