Doug, I can't help you with that, but someone will be along soon who can.
What I do know, having a '65 GMC 4000 firetruck (essentially the same thing ...), is that if you are keeping it a firetruck, you will be glad for every cube you put under the hood. Our trucks are heavy just sitting there (compared to flatbeds). My truck weighs 11,000 lbs (19,000 with 1,000 gallons of water loaded) and has the 305 V6 engine in it and in 1965, it was adequate. I guess fires burned slower back then ... ?? If you are turning it into something that will be much lighter going down the road (not usually hauling anything much), an SBC would probably be fine.
My opinion, a SBC like a 350 would be okay (but just okay)... The 454 would be the cat's butt if it fits.
Good luck.
John