320/350 small block mounts put the engine farther back than the 307/327 mounts- and the people that have issues with clearance seem to all have the 350/302 mounting setup.
i've had 2 Nova's- the first one was a 70 that was a factory 350 car that needed some "work" on the firewall with the trusty FOrd tool to get the HEI in, and one was a 71 that had the factory 307 in it that had about 3/4" of clearance with an HEI installed.
a good way to find out what mounts you have- but no a foolproof way- is to look how your headers fit around the power steering box(if you have headers). most (but not all) headers were designed for a car with a 350- so if you have a good sized dent pounded in the tube to clear the box, you probably have 307 mounts. but Dynomax headers clear just fine in a 307 car, however, so it isn't a foolproof method.
you don't plan sincerity.
you have to make it up on the spot.
wanna hear about 20 years ago when i was too smart to know any better?