going through a frame replacement myself right now for my '66 sedan, I found that the frames can be quite different depending on the assembly plant. Both my '66 cars were built in the Pontiac plant, and have identical frames (both off the cars where I can see them). However the replacement frame I have came from a Kansas plant car, and is visibly different in both the front and rear cross member areas. I seen the car this frame came from, and it was definitely another 66 GTO. I've had a look under several other '66s and seen the same type of frame in other "K" plant cars too. Don't know about the other plants, but Pontiac and Kansas definitely used different frames. Here's an example of what a
Kansas Plant Frame looks like. Have a look under your 66 and see how yours compares. Post your assembly plant if possible too, since I'm interested in which plants used which type of frames.
BTW, I paid $75 for that bare frame a few years ago in Indiana.
Ed