I am curios what some opinions for head runner size on a 418/421 would be. I had someone show me where to find a proaction full cnc 210cc that is relatively affordable. The same site has a 220 protopline non cnc that costs marginally more. By the flow numbers, the 210 flows better. Other plans on the engine are a solid roller cam somewhere around .600 lift and 250-260 @ .050. I think I am going to reuse my performer RPM intake and holley 750 dp.

I was under the impression that I would need a larger port, but maybe not. Is there a "magic" CFM # that I should be looking for?

I hope someone who has flowed an RPM will jump in but I really think the RPM is going to be a problem with heads smaller than 210s let alone anything bigger.

There is no magic runner cc for your big-inch small block.

IMO What is more important than the volume of the port is the minimum port cross-section -- or the dimensions of the port at its smallest point. There are some large volume heads with small port cross-sections and smaller port heads with relatively large cross-sections. Granted larger heads will generally have larger cross-sections, but my point is that there are some runners that use their volume more "efficiently" than others. A smaller volume head with an adequate minimum port cross-section will make great power and keep port velocity high making a better all-around power band.

Try a search for "limiting port velocity" there is a formula that will help you estimnate the correct minimum port cross-section for the size engine and RPM you are building. Of course there are other factors to consider too when picking a head, but port size (and when I say size I do not not necessarily mean volume) is a very important one . . .

Generally speaking though, for a 420" engine I think you need to be looking at heads with 210-230cc runner volumes and a port cross-section in the 2.2 sq.in. size range. It also depends on your intended RPM band, so my estimate is ball-park.

With a cam in the 250-260 range and heads in the 210-220 range
the rpm and 750 aint gonna work, they would be fine on a mild set up, but will choke your motor. You would see atleast 2" of vac. on the dyno with them