you have to understand the totality of your question..
comparing a big block chevy to a big block mopar.
is like apples and oranges...its a clique, i know..
but the though process that went into designing both of these engines couldnt be more different.

mopar uses a long rod with lots of piston dwell..
and slower piston speeds..thats whey the head ports are where they are.
chevies have short rods. fast piston speeds and this means they need a ton of volume in the head port to fill the cyl quickly..
they make power differntly ..
you cant complain about a mopar head being smaller than a chevy...dont compare it to a chevy..
i am not saying one is better...just that they are different for a reason...engineer'd
its like using a gutter pipe for a garden hose..
or a garden hose for a gutter pipe..
just the wrong application.
cheapst
now as far as which mopar is faster...? thats never gonna end..
as a teenager i owned a 396 chevelle..all my friends had big block chevelles.
the fastest my chevelle ever went was 14.60 and that was a street car..
i purchased a 340 duster(still own it) and when i brought it out...all my chevy buddies gave me a real hard time...and i mean a ribbing..
it never ended...
especially after that 340 ran a 13.00flat..
faster then every single chevelle except one in the group..
with very little mods it was low 12 sec car.
so all in all ...its relative to what your camparing to...
cheapst