There's a bit of info on David Pozzi's
http://www.pozziracing.com site on this, and I'm not totally clear on it myself as far as exact dimensions, but...
Both the motor mounts and the frame mounts are different.
The most "notable" difference is how wide they are where they fit together, one set is wider than the other. If you mix and match one way, they fit loosely, the other way they won't fit at all.
The 307 / 327 engine had ~6 1/2" balancers, the 302 / 350's had 8"
An 8" balancer engine on 307/327 mounts will possibly have the balancer hit the swaybar (think most aftermarket bars have additional drop in the center so this doesn't happen)
It's been said here many times that 307/327 mounts give additional firewall clearance, I'm not sure on this. Only time I put an hei in a camaro was a 327 mount car, and it didn't fit without some ford tool work.