I have used Plastikote Chevy Orange before with great results, excellent color match and great durability.
My brother really likes Seymour's HI-TECH ENGINE COATING - CHEVY ORANGE. He tested it in his oven to 500 degrees and it did not discolor, whereas the Plastikote did. Either one has a great match to the original orange. As mentioned above a nice high temp primer is of big help and even a splash of metal prep before that would also help with a bare block.
This is the engine in my 69 Malibu, I painted it with Plastikote 15 years ago and there is about 8000 miles on it. The photograph does not depict the orange properly, but does give you an idea of durability.
Phil Woj.