Re: 478 Diesel vs Gas?
Looking at the 478, it appears to have a smaller carburetor as well as manifold of a 305 or 351C engine, what year is it? It may actually be a non Magnum version with smaller port heads. Power wise you will definitely be better off with the gas 478, as far as the Diesel version myself I would only do a conversion to one if it were a twin turbocharged unit that GMC used in some specific applications. I have done the 478M conversion on my 68 GMC K2500 and can help you with any questions regarding this conversion.
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