With regard to reversion, most if not all engines running webers will have a point in the rev range where reversion/stand-off will occur, as long as the engine is running correctly this will not cause a fire until other factors create a condition for it to happen. Some of those factors could be a mechanical issue like wrong valve springs, bent pushrods/valves, ignition issues, wrong timing curve, cross firing in leads etc.
I look at pics of cars and many don't seal off that area around the carbs from the engine bay, this area was designed to operate as a large plenum chamber for carb air, to leave it completly open to the engine bay is to invite the hot air up into the carb intakes, hot air & vaporising fuel is a potential bomb and hot air wont improve your HP levels.
Mike I sent your info via PM.