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I came here looking for a Guru; Transmission Swap

Last week I got lucky, I found a '72 GMC BB Camper Special, it has a New 402 BB, a New Turbo 400, with all new supporting accessories, New Radiator, New Carburetor, All new Brakes, even new Brake lines, New tires, New Tail Gate, Even a new WW wiper motor, Not a Dent, One rusty Rocker panel, Just a Real nice truck, Somebody Traded it in at the Ford Dealership and when I heard the Price, I bought it without even starting it, they said they didn't have the key and one look at the shiny new Big Block and I didn't care. I went inside and payed for it.

Coming back out to see what I had bought, the Manager guy said I could start it with a Screw Driver. Some assistant manager followed me back to the truck with 2 Screw Drivers Cool! I stuck one in the ignition switch and turned, the whole lock Turned, and turned, and turned some more, after about 3 revolutions I reached up behind the dash to stop it turning, fearing for the wiring harness and................ Uh, Oh, no worries, Looking to confirm, There is no Wiring harness, Not a piece of wire in the whole truck! Ahhhhhhhhh

I went home and got my Van and Towbar, Towed it home

Well I have this '71 1 Ton 3500 Flat bed 4 Sp parts truck out back behind the house, I was always going to fix that, it's a little Moss covered and Rusty but it had the 4 sp and I like that, anyway for about 3 days I pulled the wiring harness out of it and put it in the new and much more beautiful '72, Cool, Works Great, Except the Reverse light wiring is too short, and no Transmission down shift circuits, OH! No neutral safety switch either, careful.............

The Dilemma; to, Repair the wiring harness with the addition of some small circuits or keep swapping stuff over from the other truck. I like the manual Gear. Problem is, and this is my Question, it looks like the BB is longer then the small block, it looks like it is mounted, or allowed to Extend further back in the truck, The 4 sp Truck is a small block, It Appears, if I mounted the Crossmember to the frame, and the Bellhousing to the Motor, it appears the motormounts would not line up, it looks like they would be too far back in the truck.

My Guru question, does the BB trucks have the same Crossmember?

Is it the same for a 3/4 ton as a 1 Ton?

And Just in case the rest works out, Can I use the flywheel from the 350 on the 402?