I had seen mentioned that the Ferguson streamliner had gone real high 296 or 7 MPH with a stock but heavily modified GMC head, and only went a couple of MPH's over 300 with the addition of a 12 port head.
Don Ferguson also had 80% nitro and that engine cost cover 20K, plus 30+ years of R&D. CNC is absolutely right you can make a lot of power with the stock GMC head. Don told me that one of the keys was to move the distributor (in his case the magneto)to the front of the engine and increase the cam base circle. The GMC has a big problem with cam twist and if you ever put a timing light at high RPM you will notice the problem.
