There is a difference between 307 mounts and 350's, but for the life of me I can't remember. I think it has to do with engine mounts and not frame mounts and with the right side, but that is just a guess. I think there are posts on this topic before so do a search.
The 327 or 307 frame and engine mounts move the engine slightly forward compared to the 302 or 350 mounts. I'm not sure of the exact diminsion that it is. The 307/327 mounts allow enough clearance from the firewall to use an HEI distributor where as the 302/350 mounts cause the engine to sit back too far to use and HEI without "clearancing" the firewall. I don't think there is a height difference in the 2 types of mounts.
The 307 327 and '67 350 had one set then they were redesigned for the later 350's to compensate for increased torque which was breaking lots of motor mounts back then (GM had a recall on this issue in '68-'70). They changed again as a result of a class action lawsuit to the clamshell design now. I will also conceade the HEI issue.
There was about 3/16" difference in fit. Nothing a big pry bar and a little strong arming couldn't overcome in Joe's Garage 'R' Us (unless you saw the service bulletin, you just though parts guy was selling crummy parts. I know that's what I thought at first).
Last edited by Larger Dave; Apr 5th, 06 at 01:33 PM.
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.
Actually I use a Chrysler tool in mine. Torsion bar out of a '67 Chrysler Imperial, cut off one end heated it up enough to beat it into a spoon shape, quenched it and tempered it; makes a great pry bar.