'54 and '55 have the same chassis, and factory duals were available in '55, so the manifolds will work. The tight spot is the '54 Z bar, it is a straight bar. "55-'56 Z bars have a curve to clear the exhaust pipe. '55-'56 Z bar will fit the '54. '54 bar could probably be reworked.
As for the 354 hemi, Jalopy Journal site has a wanted forum, and guys are often looking for early hemis.
John - "The Hoosier Hurricane"