[JLMounce]

Front coils aren't terrible in these cars, I'd much rather replace them than the leaf springs.

Based on your summary I don't necessarily see anything that is really out of whack. Do you have heavy stereo equipment or anything else in the rear that would change the dynamics back there? Is the car currently equipped with a lot of sound deadoning, or interior that would add weight over factory? Basically anything that would have added weight to the car that its suspension package wasn't designed around. I'm not purposely being rude, but what about the drive and passenger? People were generally much thinner in the 60's. If the occupants are carrying more weight, it can have an effect. My apologies for being direct, that can be a sensitive subject.

While the suspension does work in a choreographed manner, front springs being too soft wouldn't really effect the back end bottoming out to any great extent.

All that said, I have long told anyone who asks me that the first generation birds are woefully under sprung. Especially in the front. The back isn't as big of an issue typically because it's got a mile of suspension travel. Out of curiosity, is the bump stop still in place on the rear frame rails?