I'd bet that those mounts will only be half on that crossmember, the Pontiac frame has the mounts behind the crossmember, not on it. The member under the engine is further forward on a Pontiac B body frame in 1966 than like years of chevys. The GM divisions moved the front crossmembers fore and aft during the 60s due to their motor mount systems.
I'm sure no one knows what I'm talking about because everyone thinks that GM made all their frames the same, not in 66 B body they didn't. They, (GM) didn't standardize the B body frame until 1971. Olds is the same way, the frame is nothing like a chevy frame is.
Furthermore the Canadian Pontiacs that actually came with chevy engines, used B body chevy frames under them, GM didn't try to modify the Pontiac frame to accept a chevy engine. It of course can be accomplished, with time, money, and effort, but the crossmember is in the wrong place to just drop chevy frame mounts on the Pontiac frame. On the Canadian Pontiac there was no Wide Track because they used the chevy chassis under the Pontiac body. There's more to it than just using the chevy mounts and drilling holes in the Pontiac crossmember.
Notice in the picture that the mounts are behind the crossmember, welded solid to it, not bolted on it:
Hopefully this illustrates what I posted earlier.......
