After waiting a year and a half to receive my new pistons and cylinders for my late 71' Ambassador 750, I discovered that they were for a 72' El Dorado 850. Can I use them without changing out the block, crank, and piston connecting rods?
No. Ambassador pistons are quite different than Eldorado pistons. The diameter is the same, but the pin is 4mm closer to the crown on the Eldo pistons, not to mention the difference in the crown shape. Ambo is domed, Eldo has a low "plateau".
Did you order the wrong items or did the seller ship the wrong ones? I'm curious what your engine number is.
FWIW, you wouldn't need to change the block and connecting rods, *just* the crank.