The frame reinforcements would be the easy part, the hard part would be beefing up the suspension itself, things like ball joints and bushings aren't heavy enough to handle that kind of weight over time. Then you would have to find springs or torsion bars, which every you have, that will handle the added weight.
The extra weight will also affect the stopping and handling of the truck, so brakes, brake balance and weight distribution will also have to be dealt with.
When you take it all into consideration, it makes a stronger argument for finding a diesel that is closer in weight and size to the engine you took out.
If it were mine, I would have to consider several of the import 4 or 5 cylinder diesels, even though I'd strongly prefer to stay with an American engine.
I knew a guy a while back that took a Cummins M10 and put it in a mid 80's GMC pickup, it ran great but burned more fuel than it was worth. It turned out to be neat idea, but when diesel fuel shot up higher than gas, no one could afford to drive it.
There are a lot of medium weight diesel engines that may be an option, I would have to seriously look at size and weight vs. output if I was going to go used engine shopping. A few ideas that come to mind is maybe a Mercedes car diesel, or maybe one of the Asian cab over truck engines like Isuzu, Nissan, or Mitsubishi? Perkins also has a few lighter engines, but they weren't really meant for highway use.