Vintage race engine (273 vs. Slant 6)

If you decide to run a 273 - look into the wrist pin weights. The standard wrist pins in a 273 weigh a TON. I think they did this because the crank was designed to work with the bob weights for a 318, probably to save money in manufacturing !

You should be able to get away with a much lower bob weighted engine if you investigate this aspect. Which would lead to a faster, perhaps higher reving, more durable assembly. There might be some problems with the crank lightening process as well. But, I'm no expert on balancing motors, just a keen observer of the parts in engines.

For an intake - you might consider a 273 2 bbl intake - the 66 vintage and earlier types are single plane low rise pieces. On the other hand, are you restricted to using only cast iron ? Could you use a vintage 4bbl AL intake with an adapter for a 2 bbl ???

Keep us posted on your decisions, this sounds interesting.