Using a gasket, which goes between the stock exhaust and intake manifold [just under the carburetor] as a template, one can cut a piece of 3/8" aluminum [or steel] and drill out the three holes for the bolts which hold the two manifolds together. This plate is then drilled and tapped for fittings [usually elbow]. A new gasket needs to be made as the original one, used as a template, is designed to hold in exhaust gasses, not hot coolant. Splice this heating plate into the heater hose going to the heater core. The circulating heated coolant will heat the area directly under the carburetor.
Some have suggested that if EFI exhaust manifolds are used, and the manifolds have bungs for EGR purposes, one could make a similar plate [as described above], but pipe it so that exhaust gasses pass under the carburetor. This is how VW heated the remotely mounted carburetor on the older air cooled engines.