The Factory Assembly Manual has frame dimensions, although be aware that they're for a '54, which may have some slight differences in the rear where the pickup box mounts (there's less arch in the 54 frame). Should be good for the cab area, and close enough to get you in the neighborhood to compare with your Tahoe Frame.
Like Jerry said, starting without a title or some paperwork on one vehicle or the other could be more work than the physical conversion.
