has anyone ever put a 67 rear end into an early 70 and what were the problems encountered?

You will have to change to multi-leaf perches and re-position them on the axle tubes. You may have to change the companion flange if the '71 U-joint is larger.

I usually don't give opinions but a '67 12 posi axle is too valuable to butcher. I'd sell it and look for a '70 Camaro 12 bolt. Rare but they are around. And if the '71 already has a 8.5" 3.73 10 posi you wouldn't be gaining much. New LS1 6-speed Camaros run 12s with the Vega/Monza-style 7.6" axle.