Kind of a stupid question but forgive me because I know nothing when it comes to the frame of the truck. But cant you just fix the weak part of the frame ?
I'm guessing the air/fuel ratio valve is kaput if it is anything like the one on my 3408. I put a new one on, not all that expensive, but it took a little while to adjust it just right.
If you can stretch them, you can fix them. That is a false claim by those teaching you to look for illegal welds. What does an illegal weld look like? You would have to double frame that area too.
I think the A will stay, unless the block is no good. In which case it might get a B. The A's were prone to cracking the #6 counterbore due to poor coolant flow to the back of the head. In any case it needs a rebuild. It has a small miss at high idle and a lot of blow by. Not to mention about 3 big oil leaks at the front cover and retarder. Transmission needs to be gone through this winter as well. I can't get into deep reduction. It can be a pain to get it out of gear when shifting too, which means it could be a bent fork/rail or a twisted mainshaft.
No way I'd fit one of those in there. I don't even have enough room to fit an air to air in front of the rad.