Hi Rick,
I'm repowering a 1963 Flxible Starliner with a Cummins ISC 350HP and Allison 6 speed. You can see details at
CreativeCruiser.com
Thanks for posting pics. Please post progress reports somewhere.
Here are some other sites you might find to be useful:
Flxible Owners International
FlxForums
There's also a yahoo group dedicated to Flxibles.
Here's a shot of my bus: