exhaust pipes would be off slightly. the 300 is a shorter deck engine than the 350 which would lower the exhaust manifold in relation to the frame. i don't think there would be anything more serious than that. why are you thinking about swapping in a smaller motor? or do you have a 6 cyl now.
Swapped a 68 350 into a 67 Special that had a 300. There were no major problems. It was like it was almost meant to happen.
When I take out my 300 from my 67 Special, I would like to find a good donor car for it. Looking for a convertible. Thanks to all for the info.