Aaron,
I grew up around those old R-Models, the company my Dad drove for had about 20 of them! The first semi I ever drove was a '57 with tandem rear ends. I was 14 yrs old! It had a 501 Red Diamond straight six in it, with a five speed main transmission and a three speed auxilary.
I'm building one myself, and I'm planning to use a Cummins out of an AMT kit. The engine color was red on this engine. I think the V-8s were red, also. The big V-8's I've seen were 549 C.I.. The engine out of the C-Series Ford or even the Louisville would probably work, since you shouldn't be able to see the top side of it.
Here's a link that may be of help:
http://www.oldihc.org
I checked out your website. I like the fire and rescue trucks you've built! Especially the older ones!