This swap has been done, and some of those who have done it claim it works real well. The frame is only a little wider than your stock frame and getting the body on it would not be a big challenge. Aside from being a lot of work, the problem is that the width of the axles is wider. The wheel mounting surfaces are enough further out from the center of the frame that your challenge, especially in the front, will be getting the tires under the fenders.
Rather than clipping the frame a number of people have removed the IFS crossmember and bolted it to the older frames. In the area where the crossmember needs to bolt up the newer frame is about 1-1/4 inch wider than your '49 frame. I've seen the swap done by narrowing the crossmember, which helps with the track width problem, and by using spacers between the frame and crossmember.
From what I've seen the touchiest part of this swap is locating the steering box and idler arm correctly. If you do not position them in relation to the crossmember just as they were on the donor truck you are likely to have bump steer problems.
The advantage of taking the clip approach is that you can keep everything in its original location. The disadvantages are the same as for any clip project.
If you search my posts here on the Stovebolt you'll find a thread where I asked if anyone had one of these IFS swaps that was finished and on the road. At the time no one responded who owns a finished and driving swap of this type.
You will find reference to Steve Andereggen Jr and his swap. Steve Jr had bump steer problems and has since gone back to a straight axle on his project. I've met Steve Jr, seen his truck and his pictures, and feel I understand the problems he encountered. From his experience I emphasize careful placement of the steering box and idler arm.
There is a truck I've seen on the road actually running down the freeway with this swap. It took some doing but I found the owner and managed to spend a few hours with Joe talking about his swap and how it works. Joe is a guy in his later 70's who does not own a truck newer than 1950. He tells me his drives just like a new truck. I'm a little sketical of his claim because I'm not sure he has ever driven a new truck, but the evidence all says he has driven his tens of thousands of miles over the last ten years, and that his swap was successful.
Joe had wheels modified for the front of his truck and they still stick out about half an inch outside the fender on both sides.
Just recently I met a guy who lives only a couple miles from me who put the same IFS crossmember under his truck. His is a dually and by using 1/8th wheel spacers he was able to fit the stock 18-inch dually wheels on the IFS and inside the fenders. He hasn't finished his project and has only driven it a few miles. He does like the way it drives.
I've not gone back and reread this thread. I'm sure I've mentioned already that I am building a 1-ton to pull a trailer. I'm using a '55 1st GMC frame with a 14-bolt GM rear and a '72 C20 IFS. Today the 14-bolt is mounted but the IFS is five feet away and I doubt it will get any closer during May because of other commitments I have. I'll run a 500 Cadillac in mine, but conceptually we're doing very similar things.
My front wheels and tires will stick out too far. My plan is to mock up the cab and fenders once the IFS and motor are in place, choose wheels and get them in place, then modify the fenders to cover. I expect to use four fenders to make two, basically widening the fenders an inch or two on each side to get enough width. This will not be a trivial exercise.
I'm not convinced the S-10 frame is not stout enough for your propose, but would agree that the suspension and brakes that come with the S-10 are not adequate for towing the kind of loads you want. Changing the rear would be relatively easy. It may be possible to increase load/brake capacity of the front but I've not heard of anyone doing it and don't know how to approach it. But I don't like to extend my ignorance of how to do something to a conclusion it can't be done.
