Vintage race engine (273 vs. Slant 6)

Thank for the input. When I posted, I cut out a lot of the extra rules as I'm just trying to choose between the two engines. Classes are based off # of cylinders. Cars up to 1965.

I looked up the LD4B and I see that its a 4BBL . As the rules state 2BBL for V8, I don't think that'll work.
I have found a source for the 4BBL pistons for the 273, so that would raise compression to 10:1.

As for /6, Offy makes a 2x1 BBL and a 4BBL manifold. I doubt I would ever find a Hyperpak and I doubt I would want to pay the price if I did.

Rapid Transit, yes I want run the Carrera Panamericana. For those who are unfamiliar, it starts at the bottom of Mexico and ends in Neuvo Laredo. It mostly runs through the mountains of Mexico and cars rarely exceed 85mph.
So, some fuel efficiency and torque could help.