"I'm having problems finding local sources for 403s as its a bit of an oddball engine since it was only offered for a few years and I was wondering if it might be easier to hunt down a different Olds small block"
That is a pretty heavy car, the the extra cubes and torque are nice to have. A 455 is taller, so the other choice would be a 350. No real sense in losing the extra 50+ cubes, and you already have it!
What issues are you finding as regards to parts? What is your over-all goal for the car?