Unless the frame is junk I wouldn't want to plan on changing the fame. That is especially true if you plan to keep the long bed.
The car frames just flat don't work right under these trucks 99.9% of the time and usually end up being a cobbled up mess in the end. I've seen a few exceptions over the years but the builders were pretty creative fabricators.
I'd suggest buying a front suspension kit for the front that was set up for bags and having the rear of the frame C notched and set up with one of the link kits available with bags.
Scott on here can probably offer up the whole setup front and rear that your guy can install in a reasonable period of time. It won't be cheap but you will have a world class setup when you are done. Or you aren't all that far from Fatmans
http://www.fatmanfab.com/ They do do complete suspension installations in their shop along with selling kits. It might be bit of a ways to haul the truck but the turn around time should be pretty good as that is what they are set up to do. you would probably want to do the prep work before taking it up there.
Engine choice is what you want and not what any of the rest of us would want.