from any of the various 55, 56, and 57 Chevrolet kits that Revell has put out. I "fixed" my problem with that by putting on the Chevrolet Power valve covers.
What a lot of people are doing in real life is stuffing a 350 in the Novas. A neat one to do would be the LT-1 from a Revell 69 Nova. Even better would be that engine with the dual quad cross-ram from the 69 Z/28 Camaro.
Just remember to use the Moebius oil pan on the 350 for easy fitting.