then only thing that is strange is the mount you did not say if the car was a ambulance or combination. but guessing it's a straight ambulance as it has a permanent mount.
for simple you cant go wrong with the 17 to make a permit mount you will need to drill 3 holes for the mounting rods. some were I do have a set of pictures showing the mounting of one.
but in reality if you can access the hot wire you could use what you have and let the skirt cover it all.
federal used the same guts, skirt and globe on all there lights in this series. 17,173,174 and 178 the pipe mount has the base housing that the roof mount did not have. the demountable used a base plate and hollow tube that the wire went threw and it went on with a wing nut.