Bad news - GMC and Chevrolet turn signal switch are not alike - they are numerous differences between the GMC and Chevrolet trucks built prior to the early 1970's. Don't reply on much help from parts vendors and assembly manuals that lump GMC and Chevrolet together -
My advice is to save everything you can from your wiring such as ends/terminals and scout around and find parts/pieces of ORIGINAL to your truck and go from there. The aftermarket part sources are junk - think China.