That's not accurate.
For '69, Chevy used different motor mounts and frame brackets for the 307/327 and 302/350 engines. The 307/327 parts were carried over from 67-68. 302/350 frame brackets/mounts were a new design, incorporating an interlock feature. The frame brackets are shorter and narrower; the mounts are taller.
The problem arises when 67-68 motor mounts are used on the '69 302/350 frame bracket. They fit but are wider. It is a sloppy fit, and will allow the engine to move around, causing perpetual clutch chatter and some fitment problems as the engine will be positioned too low.
Each design places the engine in the same position. Don't mix the parts.