Slowgoer,
Most likely, you'll get a few conflicting opinions on engine paint colors. Gray was the color for 235 (and I think 216) truck engines (car engines might have had different colors - 235 Blue Flame 6's were blue). The 261 engines were green. Jim Carter has engine paints (gray by the pint or green by the spray can). Chevy Duty sells a blue paint for truck engines (and did not offer to take it back) - I'll give you a pint can as a Christmas gift if you pay the postage.
What engine color did you find?
Here is a brief reference on a website (I think the BOB is Bob Adler - he is an expert on Advance Design trucks):
http://www.chevytrucks.org/tech/paint-colors.htm I have heard some people say that whatever paint was around was what was used on original engines.
The firewall is part of the exterior cab and is painted exterior body color. Inner front fender "walls" are black (as is the radiator and radiator frame) - a satin or flattened black is most likely correct (Eastwoods sells an underhood black but I think an epoxy would be more durable).
I do not know the color for the little filler panels that screw on the firewall to close the gaps between the firewall and the fenders.
Tim
