340 Intake Manifold

68-70 are squarebore like the one in the link I posted.
71 and up are spreadbore for the Thermoquad carbs.

A stock 340/360 intake is probably better then the Performer bacause they have larger ports. The Performer intake I think has the small 318 port size like the LB4B has.

If I had the choice between a Performer and a stock iron 340 intake I would run the stock one. However your stock 73 intake wont work with a edelbrock or Holley carb without an adapter.

I bet the LD4B out flows the Performer also.