The Hedman shortys aren't tuned length, so that limits their benefit theoretically.
I don't have hard evidence/data, but after I put shortys on my C-body (it had standard logs) the performance increase was impressive, and I don't imagine HP manifolds would've made as big an improvement.
Depending on your year of C, the HPs make a sharp turn at hte steering column (69-71 cars), so keep that in mind whilst trying to decide.
Either way, those shortys just look like they would flow well. They'll come with 2 1/4" collector hookups, though (
) so plan on something better. I welded 2 1/2" pipe right to the mating flange and bolted them together. No problems, several years later.
IIRC there was a Mopar magazine that did a dyne comparison several years ago.