best hi flow ex manifold/drivers side

Big bucks if you can find one.
A cheaper alternative would be the '92-93 Magnum manifold that has a larger outlet diameter than the later ones, casting numbers 617/618. Even the standard Magnum manifolds don't give much up to the larger bore ones or the 340 manifolds.
They all will take some horsing around to get them to fit an early A (even the 340 manifolds need the column sleeve cut back) but it can be done. Here's a thread on doing it with the Magnum manifolds:
YES 5.9 Magnum manifolds work in early A bodies. Here is exactly how!
And BTW, for anybody reading this in the future; none of these work with power steering.