bnxbombers

I have 6 into 2 header w/2.5" exhaust (MSA) on my mostly stock '73 (SU's). The car runs fine - other than a lumpy idle. It makes nice noises and doesn't miss/surge/knock. But as I wind through the gears it flattens out above 4000 rpm. No power gain at all over 4500. it has done this since I've owned it, 10 years. I have had the plugs/points/filters changed, and the timing and carbs adjusted by a quality shop in NJ. Electrical system is putting out almost 15 amps. When I bought the car it came with an extra head/cam. I'm wondering if the prior owner had swapped the head/cam and somehow made it worse than stock. Any ideas? Also looking for a shop in Orange County CA that can work on the carbs and knows Z cars.