A big can of worms is the title and vin. Everyone has their theories and methods on how to overcome this, just make sure you make a plan that's going to work for you and the local DMV when you register it and then possibly try to sell it later.
Better advice would be to consult the DMV (or whatever it's called in your location) and ask them about this kind of thing.
Some areas allow transfering the VIN plate, some areas require the title to be reissued with the VIN of the salvage cab, and some places will require titles for both cabs be presented at an inspection, where a new VIN will be created for the vehicle.
Going about this the wrong way, can land a person in jail, or having a vehicle siezed which is then crushed and/or parted out.
Doing a little research beforehand can save a lot of time/money/greif later.......
where would the frame vin be? i was just planning on registering it under the new cabs title. ive never really known new york state to go by a frame vin.
Going by the replacement cab's vin is a good plan. Not many people know about the frame vin, just the one staring at them under the windshield. You would not be doing any grinding and swapping plates, so you can avoid that legality problem also. Your cab door jamb sticker will probably not match the way the truck is equipped, but I would not worry about that either. You will have to pay the registration to get the title swapped in your name(you are after all getting a new 86 truck correct?) and get the tags swapped, and you may have to pay a little bit more in taxes since the 86 is probably worth more than the earlier truck.