Normally, an engine can handle some exhaust restriction because of the high exhaust gas pressures.
I am not sure what you did, but the 45° angle the header leaves in the picture reduces the exhaust area by about 30%, and that may well restrict exhaust flow in the high rpm regions. A larger prop therefore might do a lot better than your medium range 24x10 which needs to rev up.
Test the engine on a test stand with a more open header, and you will know. All else on your rig seems to be OK.