I was just wondering do all '64 thru '67 rear bumpers interchange between El Camino's? I figured they would since the frames are similar, etc. Trying to open up my options for the '67 elky.

No they do not interchange. Actually each year is different. They do interchange with the same year wagon though.

Isn't the 66 and 67 the same 'mounting' but the 67 has slots/openings for backup lights???