Welcome to the site! '55 and '56 'Bird wheel rims are the same as Ford passenger car rims of those years, Mike. I don’t know if ’55 and ’56 Ford wagon rims were wider or not. Another Forum member will know for sure.
The '55 and '56 rims I’ve seen have…
- a center hole opening of 2 9/16, almost 2 5/8 inch diameter...
- 5 lug nut openings...
- 5 flat faced ‘spokes’ between the 5 lug nut openings...
- the letters FoMoCo with a surrounding ellipse stamped into the flat 'spoke' surface adjacent to a lug nut opening...
- a 3/16 diameter hole opening on another of the previously mentioned 5 flat ‘spokes’...
- the letters K-H stamped on yet another of the above mentioned 5 flat ‘spokes’...
- 3 ‘lugs’ that could be used to secure hub caps (not wheel covers) if used
15 inch tires...
- a measurement of 16 ½ inches from perimeter edge to perimeter edge of the wheel rim diameter...
- a measurement of 5 inches between the rim surfaces where the tire bead seals...
- have (4) 5 inch long vent slots where the center wheel mounting plate fits into the inside perimeter of the wheel rim...
- the wheel center mounting plate should be riveted to the perimeter wheel rim –look for three button head rivet heads on the backside of each of the four center plate mounting ‘tabs’
Notes:
- there were aftermarket replacement rims from other manufacturers that will fit your 'Bird or ’55 and ’56 Ford cars, BUT they will not have the FoMoCo stamping or the K-H markings, may not have the 'lugs' for securing hub caps, may have spot welded center plates instead of riveted plates, or other descriptors mentioned above.
- the K-H stamping is usually quite clear; the FoMoCo lettering and surrounding ellipse stampings may be faint due to the depth of the original impression OR the paint thickness that was applied…
Hope this info helps. 
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York