I just replaced my gear in my '37, Shine is right they (the 8") fit right in, I have always been under the understanding a 56" rear measured from where your wheel bolts to to the other side will fit most Fords earlier than '55 as thats what they ran due to being common with wagon spacing. Now the 8" gears are much higher and not as many ratios are available but they are plenty stout for the street, and the axles are the same as the small spline 9" rears, if you can't find one with the right gear expect to pay a extra hundred over the 9" for the gear, the real benifit to the 8" is they won't need narrowed (for most early rods) but if they do just buy a 9", also generally wether the sales ad mentions it or not most 9" brake kits fit the 8" as they use the large bearing style flange the same as the 9"
I agree with Roger, hit your cruise nights and ask, I would rather do that than simply pick the person with the biggest advertising budget which many times translates to the highest labor rate per hour, not a real difficult job for a experienced shop