Before you get into a lot of fabrication work and expense, it might be wise to check into your state's procedures for getting a title and registration for a vehicle assembled from salvaged items. That's particularly true when it appears that neither of your donor vehicles seems to have any sort of paper trail. Some DMV people get downright unfriendly about that sort of situation.

Next- - - -how close to the 51's wheelbase is the Tahoe? If you get into cutting and stretching or shortening the new frame, you'll have to deal with altering the length of the driveshaft, brake and fuel lines, and a bunch of other nit-picking details. Make it work on paper before you grab the wrenches and the welder! Have fun!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!