What most folks forget is that airflow changes with volume and velocity with the addition of length of the intake manifold. Many years ago, I worked with EFI manifolds on my flow bench trying to answer your question to my satisfaction. I found that the airflow cfm in a head flowed at 28" would pull down quite a bit when bolting the intake manifold to the head. I also tried it some by checking the difference with different lifts of the valve, and was surprised how much things change. It is VERY difficult to get a manifold to bolt up to a head, and the flow NOT drop a little/lot with a cast manifold, especially dual plane intakes. The FE is one of the hardest to get the flow to stay up because of the turns and twists of the intakes with the exception of the single plane intakes. A sheetmetal intake is designed with the idea that you not only can match the head flow, but actually increase the volumetric efficiency to upwards of 125% as in a Pro Stock engine. To get the FE engine to perform at the maximum potential, you will find the best engines use sheetmetal intakes. The reason I came up with the 110% street/125% race is directly related to flow bench testing, and David Vizard's formulas in his flow bench booklet from the '80s. Lots of folks don't like DV's attitude, but he is successful with his builds, though now things have begun to pass him up with all the computerized flow simulations, and CNC porting. The fact is, though that if you look at what works, line of sight, taper, parallel walls, and port volume all help to fill a cylinder to maximum. A manifold should not pull the head down much if any, and it takes a lot of work to get all the runners to flow close to even in the FE dual plane intakes. Edelbrock has done a great job with the RPM, and the new TFS Track Heat is simply the best single plane available out of the box. They can be improved upon, and not lose performance with the right port work. The Blue Thunder intakes are very good as cast, but have a lot of variance between the different ports in flow cfm, but can be corrected with flow bench work. The BT 4V can be made better than the RPM with work. Having said all that, most folks don't realize that the original FE heads like the CJ only flow ~250 cfm, and the PI intakes 270 cfm average, as cast. The Sidewinder is less than 20 cfm better, and the RPM out of the box is ~310 cfm average. Very few folks have the ability to actually flow intake manifolds and very few actually know what their work does flow. Most just gasket match, and if it looks good, sell it. I have flowed Wilson CNC'd manifolds that had a 80 cfm difference between their runners, and cost $2500.00 for their work. Joe-JDC