As John said the center dump ex manifolds called ramshorns (and they look a little like the scrub's ex manifolds of the same name) will not work on a stock suspension pass car. I remember reading someplace, probably one of Ted's dyno tests, they don't make any more power than stock '57 and up pass car ex manifolds. The '57 stock pass car manifolds will fit in a 54-56 OK but the head pipes are a little different. The Ramshorns are a great fit on every truck I ever saw and they look really cool.
But for real power - normally aspirated, I think most would agree headers are called for.
Re ex manifolds be careful of the ears on either end. Actually FoMoCo intended that ex manifolds of all kinds fit directly on the head without gaskets!! Gaskets tend to cause the ears to bend and as CI doesn't like bending, the ears break.
Real gurus ie Ted et al, any comments on ex manifolds with or without gaskets?
PS those could be Jahns cast pistons. Being .040 over they are clearly replacements and I think Jahns tended to have the machining mark in the center.
Doug TThe Highlands, Louisville, Ky.