Big brain question

I've switched all my trucks to elog exempt and now it's my turn. I know I know some people hate older trucks and would die without their automatic Volvo 770 that gets 10 miles per gallon at gross going up hill. The rant is done. Now for the question. I've found a 379 at a salvage yard. The whole powertrain has been sold and is gone. I've always driven the responsible truck. Now I'm going for old school rooster cruiser. Stretched frame and all. I know you can stretch a frame but I want to go with a new frame. The hood on this truck is cracked. So it will be replaced. The truck has been sitting in grass nearly tall as it without tires or rims. So I'm sure it's full.of ants and bugs. So I was thinking that I'd just buy a cab and sleeper that's in better shape. If you've been keeping score you realise that pretty much everything with be replaced except for the rear ends. Question. On paper as far as the vinegar goes will that truck be the older truck on the cab that I buy? The reason for changing everything is because I can't find anyone that does frame off restorations on trucks they aren't doing for themselves. I've even call the shop in Joplin that used to do chrome shop mafia. They don't do it anymore. I haven't bought the truck yet. I'm just trying to figure things out in advance.