The 150s and 250s are the same and 400s are longer. When your saying "stands", I assume your meaning the ceramic sockets. IMO the only way to mount sockets it to use a bulb seated properly and then mark the holes. If I give you a measurement and then you mount them, if its even an eight inch off your either starting over or moving one socket. Its almost impossible to move anything just an eight inch.
So, in short, just buy the bulb you plan to use, and use it to make your template.