I hate seeing old trucks get modernized. They can be done nice and clean and not looked hacked up but it takes considerable amounts of time, money, engineering, and fabricating. However if you do decide to go through with the project you will have to consider a few things.
* The 73-87 square bodies have a drop center frame that will stick way below the cab
* 73-87s have a wider track width
* The firewall will have to be clearanced to have enough room for the V8
* The '46 pedals hinge on the floor. In order to utilize the power brakes and split master cylinder the brake pedal will have to hang from the firewall.
* You will either need to fabricate a clutch linkage, convert to hydraulic, or if it's an automatic buy a floor shifter ($$$$)
* The small block V8, power steering pump, power brake booster and master cylinder will take up considerably more room in the '46 engine bay. Everything must be carefully laid out beforehand.
It can be done with careful planning (and deep pockets) however I would opt to keep it as original as possible. In the end it is much easier to stick to stock than attempt a project of that magnitude, especially if you want to get it on the road quickly. I'm not trying to be Billy Buzzkill but I would hate to see you tear apart then give up halfway through the project. You could also consider transplanting the motor and tranny. That solves the running problem and if you make new crossmembers instead of hacking up the old ones it can always be converted back to original later.
just my .02
