chassis dyno results

I had my 73 dart on a chassis dyno this weekend. I have a 446, 9.98:1 compression, Mopar .557" lift cam, roller rockers, Eddy 84cc chamber aluminum heads, M1 intake, Holley HP750 carb, TTI headers and good flowing exhaust, 727 trans built with a 2400rpm converter, and 4:56 gears in the rear. Here is something unusual, the dyno checked my car as only making 308 hp at the rear wheels, I thought sure it would be more like 375-380. The pull started at 2000rpm, and ended at 6200 rpm. The odd thing is it made the same 308hp from 2400rpm up to 6200rpm, give or take 2-3 hp either way. Do I have a converter or my 3rd gears clutches slipping? It has to be something like that, doesn't it? Plugs look good, and car runs excellent, except for a slight lean condition just off idle. (I am sizing air bleeds down to fix that). Any ideas?