Have you considered using led lights simply because dichroic lamps like the ones youre thinking of get really hot and always burn out. If youre fitting from scratch you can retrofit or get entirely new pods. I use LED spot warm white in my kitchen and they are about 9watts each...the light spread is about right because I picked a wide angled beam.

Here is a link that might be useful: LED strip

The answer to your questions depends on the dimaeter and type of fixture you are using? Is it low or line voltage? Will the ceiling be insulated?

Recessed lighting is usually spaced on a 16" module so 40" is odd since it would probably put a light on a joist or strapping.