The frame is the original R120 frame. This frame was in good condition when I got the truck back in the 1980s. It had developed a few minor, but, repairable cracks at the front. This is typical for these frames. I built a strengthening plate/brace to hold the Saginaw Power steering box. Because of the weight of the 6.9 V8 Diesel I added air springs to the suspension, both front and back. I put the front air springs directly under the engine mounts. This was for direct weight transfer to the front axle. When the engine mounts are not directly over the axle, the frame is forced into extreme flexing, from the "Z" angles, causing the frame to crack and break. This was typical of these old IH trucks.
I got 225,000 miles and 20 years on this frame and there was no cracking.
These Old IHC frames are easy to work with. I would never consider using a Non IH frame, unless I were building a new frame. these frame are a simple ladder design and easy to make.
I installed a 1990 Chevy/GM Van fuel tank in the frame rails, at the rear of this "R". A good trailer hitch is mandatory for added frame strengthening. Without this hitch or something else to add frame strength, the frame will twist and ultimately break along its length.
Another situation you might have is the deuteriation of the spring perches. I chuck the perches in my Southbend 16" lathe and drill out the perch pins. New pins are easy to make. I have two styles of pins. One is external thread, like the originals and the other style if internal threads for a bolt.
The better choice for an engine for these trucks is the Cummins "B" series. The 6bta, like a Dodge truck will fit without any surgery. I have a perfect Cummins 4bta for this L110. The Cummins 4bt looks lost in the engine cavity.
https://youtu.be/_U5M6eIQpZk