Are you sure the frame mounts are correct? The '67-'68 frame mounts moved the BB forward from the firewall and tilted the engine slightly to the right for steering box clearance. The GM PNs are #3912597 for the driver's side and #3912598 for the passenger side. They've been long discontinued, but if perchance you scored old factory mounts this may help you identify what you have. There are 2 different early block mounts, too--one has a thicker rubber insulator that would raise the height of the engine, but I can't remember the applications.
In the '70s I did this swap in a '69 for a friend--307 to 396--and everything from the small-block worked, including the exact same mounts--frame and engine. I warmed the 396 over with a .550"/306 Crower cam, headers and a Holley, and that thing would smoke the tires out of sight!
Hope this helps.