If it were me, I would put the 75 body on the 4x4 frame. Put a body lift on it so the transfercase will clear the 2wd cab
The VIN is on the cab in the door frame. If you put the '75 cab on the '78 frame, it should register as a '75.
California is a strange place when it comes to registering a vehicle.
Hillbillydrums, I have a few questions I need answered before proceeding.
Is the 75 and/or 78 still registered ? If not how long have they not been registered ? This will matter greatly when it comes time to get one registered.
There are Lot's of people who are doing what you want to do and do it legally. Now if you do not want or care about getting the
CHP involved for inspection purposes, then there is also a route to go for that.
I am doing the same kind of thing with my 74. My body was trashed and the PO welded 1/4" plate all over the place in the cab and on the sides of the cab. So I found a 77 cab with AC that I am using. (You have to document everything for this kind of swap, or if you don't care like some I have seen do it, you just swap vin plate tags when you are done.)
To answer your first question.
YES you can swap the 4wd parts onto the 2wd frame. It will require complete tear down and drilling out the rivets that mount all the suspension parts and when installing the 4wd parts using grade 8 bolts.
The much easier way is to use the 78 frame and all the running gear and swap the 75 body with a body lift as Zeig suggested, or modify the hump if you do not like the body lifts. ( You can do this the legal way and go to DMV and tell them that you are doing this and using only the cab for whatever reason you want to give them ), or like I said you can just do it and tell no one and change the vin # tag, which by the way I have to state for certain reasons
is totally 100% illegal in California but happens all the time and no one is the wiser.
Are both trucks manual or both auto, not 1 manual and 1 auto ? This would create a little more of a challenge. There is also differences in the interiors between the 2, small but noticeable to a trained eye. 77 was the cut off for some of the body changes also, I believe door shells and things like that.
Sorry for the long reply, but I have done both ways in the past and if done right no one knew any better, though you should never break the law. ( for attorneys sakes

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