The 350 may not have provision for front/corner mount brackets.
The 350 will have the provisions for the front mounts. All SBC engines had this provision, but the early ones didn't have the side mount provision, as you said.
As you alluded to though, the frame portion of the mounts may be different between a 6 and 8. They were in my '62 Bel Air, so I needed frame side mounts from a V8. However that was a side mount installation.
