Horse power drop using early A 273 manifolds

273 said
Installing Exhaust Headers Into 300hp Crate Engine - Mopar Muscle Magazine

On this 310-320 hp 360 there's barely a difference between 318 manifolds and headers, I highly doubt 273 manifold are that much different than 318's.

The ports are the same and the outlet diameter is about the same as later manifolds, the problem on the early A manifolds is where the dump flattens out to curl around the steering column. It looks VERY restrictive, much more than the later A manifolds. That's why some real world numbers would be interesting...
I stole this image from dgswinger's for sale ad to illustrate the bottleneck.