is there a difference between sbc and bbc frame mounted motor mounts and what is it?

Only the 307 mounts are different. All others (BB &SB) are the same.

I've never seen those.. Are they an application specific mount or an aftermarket item??

I don't know what year you have but '66 &'67
frame stands for big& small block are different the big block ones are notched different to allow the interlock tab of the bigblock engine/motor mounts.

Both keep the engine in the same place.

Dana it helps to let us know that you own a 66 as there are differences with body style changes.
Brett wins the prize for the correct answer, but the small block mounts physically will work but are not as strong.

thanks to everyone for the info.very helpful....66 300 deluxe