Why does he want to put in on a newer frame? I don't ask that to challenge the decision, but because understanding what he wants to accomplish is likely to change my thoughts on the best approach.
I have one with a nearly stock frame and suspension and in fact drove it to work today. I've changed the rear axle to provide more highway friendly gears, and it has a 'modern' disk/drum braking system. I put modern in quotes because I know not everyone thinks '73 Chevelle parts are modern. Aside from those changes and new bushings and stuff, the frame and suspension are completely stock and I'm happy with it the way it is. At least for now...
I'm working on another which is going on an S-10 frame. I considered several different directions on this particular truck, and after starting down another path have come to the conclusion this is the best route for how I want to use this particular truck. The primary objective with this one is to create something my wife will be comfortable driving. After a couple decades of providing her vehicles I have a real good idea what that requires.
At the same time I'm preparing to start yet another project, this one to create the ultimate classic trailer puller. Starting with a 1-ton '54 GMC I want to pull a heavily loaded trailer with comfort and finesse. I haven't settled all the details yet, but just a couple sentences describing how I want it to perform and how I intend to use it make a lot of difference in thinking through what changes make sense. I know it will have a big V-8, a Chevy 454 being the smallest I've considered, but I also have a Cadillac 500 that may go in this one.
What you or your brother-in-law mean by 'hot rod it up' may be clear to you, but to me it could mean many things. Each of the trucks I've just described qualifies as a hot rod in at least some respects, but each is completely different under the sheet metal.
Respond with a couple sentences describing how he wants to use the finished truck and our advice will be that much more useful.
