Normally, a D1-4 backplate (which is what I assume you have but it could be a D1-3 -so better check) the taper part that mates with the spindle will be very tight (undersized) and may need the angled bore widened-up. This is often the case. Once that is addressed, yes, you mount the plate and surface the chuck-mating side.
It's also recommended that you make a static balance checker and mark the heavy side of the plate then, put a bar in the jaws of the chuck alone and find it's heavy side. When you go to drill the holes through the plate, you mount the heavy sides opposite of each other and that way, the overall unit will likely spin with a more neutral balance. Once they are mounted together, you check the overall balance and make divots in the backplate to make any further corrections.
Here is a thread that covers the entire procedure. If you are new to using a lathe, this may seem a little daunting...
Here is a thread that describes the process:
http://www.hobby-machinist.com/show...Spindle-amp-Chuck-Measurement-and-Setup/page2
Ray