I have a 1971 Monte and in the middle of doing a big block swap. I just pulled the 350 out and notice the existing frame mounts for the 350 look like they can be switched to opposite side to move mounts inch or more forward to accommodate the big block. There is a hole not being used on left side the will be used on the right. It looks like it was meant for a swap. Is this my imagination?

 

Maybe the Maniac can ask his buddy on this swap , but from what I understand is the mounts look the same but there is a slight difference between the two.

 

According to RockAuto, they're the same part number.
You should consult your 1971 Chevrolet Service manual for proper installation.