You need to post some pictures of your engine - top and bottom. There may be an easily spotted issue - missing engine tin, unplugged heater tubes, missing air seals around the plugs, no Hoover bit, missing deflectors around cylinders, exhaust gases free to enter engine, missing tin/air leaks around dog house cooler (bottom piece is often missing), aftermarket shroud with no internal vanes, chrome cooling tins, etc.
Most VS engine bays I've seen have a round ~5" air inlet in the firewall (behind fan intake to right a couple inches) covered with a mesh screen - does your have it? Note - VS in photo also has rubber seal completely around the engine lid - great for keeping rain water AND extra needed air out. Other have made a prop that keeps engine lid open a couple inches when driving to let in extra air.
Late 1975+ FI bugs and 356 Porsches (first used on the 4 cam 550 engine) added something called a Venturi ring to smooth/increase air flow into the engine cooling fan/shroud. From AirCooled net -