Oh Boy!
The first question is fairly easy: Firewall is body color for all Chevy trucks. Period.
Inner fenders should be "semi-gloss" chassis black.
Engine? 'Sup to you; 261s were green, I beleive. Are you doing the "two-stud valve cover conversion trick" and trying to make it look like the stock 216?
If you're trying to disguise it, then paint it "Stovebolt" gray.
If you're proud to have that 261, then I'd suggest painting it a "correct" 261 color.
Personally, I have two 1954-55 1st series truck that have Black engines (probably re-builds), and I think I'll probably repaint 'em that way... it's not "correct", but the one truck was my Grandad's, and I'm going to keep it the way it looked when G'dad had it.
Have fun; it's only paint!
