66 Valient, Exhaust Manifold Options

Depends how your car is optioned.
Power steering or column shift pretty much require use of the factory early A driver's side manifold (but will require grinding or an exhaust spacer to be used on later model heads).
Manual steering and floor shift/pushbutton shift, you can fit the late A (including the HP manifold) and the Magnum, although they may (will) require trimming the outer column back a fair amount, and the Magnums may require a bit of additional grinding for column/coupler clearance, and/or a change to a steering u-joint. Some people have also angle milled the manifold face to gain the needed clearance.
Most all passenger side manifolds (both center dumps and rear exit) that I have seen fit fine, some tighter than others, but nothing that can't be cured with a little creative hammer work to the fenderwell or grinding of casting ribs on the manifold.
There are header options, but I won't get into those since you specifically asked about manifolds.