The supposition then is that you are running a 351C, is that correct? If you have a 351C, are you running the 2V or 4V heads? If you are not for sure look at the top corner of the head - you should see a raised 4 or a raised 2 in the casting.
The Edelbrock Performer Intake for the 2V should say right on it, 351 2V. The ports are not exact but are close. If you have a 4V head then the ports are much larger. There is a specific Performer intake for the 4V heads.