You are correct- they do look almost identical on the outside. The differences are in the inside - specifically port /sleeve timing and locations. The Q40 422 guts won't be happy unless you are spinning a smaller prop in the low to mid 23k rpms. The Q500 428 guts will want to see around 19k rpms. There are specific props for each motor.
With the death of 428, being replaced by 426, there should be a lot of Nelsons to be had. What would be wrong with the radial mount? Really convenient actually, even in a sport set up. Or is this for display?