I have run dual carbs on a couple of different manifolds over the years. I did not use a insulator/spacer. I did, and still do, apply heat to the under side of the intake via hot water from the cooling system (if you are not using the stock exhaust manifold). To do that with the offy, you have to flip it upside down. Where the sheet metal core plugs are located, you have to weld little plates over them. Then make up a bolt on plate or weld a plate to the bottom where the stock exhaust would have usually gone, with two fittings to run hot water from the thermostat housing to the manifold, then the manifold back to the cooling system. Some gmc truck lower thermostat housings, like 302 and 270 gmc motors, have a threaded port on both the driver and passenger side of the lower housing to make hook up easier. You can also make up an aluminum spacer to go in between the lower housing and the head with a threaded port in the spacer (also gives some room for aftermarket aluminum valve cover so it can be removed easier.
