I have a 79 bronco with a stock 400m wih a 425 impco propane setup.I remember reading somewhere that it is a good idea to block the "heat riser " to keep the intake manifold cooler, but I could not find any more info ie pictures or a more detailed description. I have the manifold off now to redo the head gaskets and would like to find out what is involved in doing this modification if it will gain performance or mileage..anyone know anything about it ??????
With your 400 (no M) it is easy to do. All you need is a bunch of those copper kitchen scrub pads available from the grocery store. Pack them into the heat riser passage in the manifold as far as you can from both sides with a metal rod. You can also stuff them into the head passages. I have heard there are also some gaskets that close the passage but they might rot out over time.