I dealt with this as well.
I converted a 240 to a 300, rear end and all.
I finally discovered after trial and error that the way to get the speedo you want is to find one that has the shift ticks on the same spot for the ratio you have installed. For example, The shift ticks on the speedo face were different on the 240, than on the 300, compared to the gasser model 123 series (which I bought by mistake thinking it would work and that the 140 mph would have looked cool).
As the other gentleman mentioned, the rear end ration determines what speedo head it has, but a quick way for you to determine which it is you need is to determine where the ticks are on the speedo face next to the exact mph for where the shift is supposed to occur.
I hope this wasn't too confusing,
Adam