The first thing to take into consideration is the frame ramp is cut for the 45 and not the 40, this can cause feed issues if you are wanting to use a standard barrel.
To solve this problem the frame can be milled for a ramped barrel. I highly recommend barrels with the Para style ramp, because of the radius design.
If you want the most reliable gun it would be best to build a complete 40 topend, slide, barrel, firing pin, recoil system, and extractor.
Also it will be best to change out the 45 ejector in the frame for an extended one designed to work with the 40.
I will say once you start adding up the cost of doing the above you will most likely be able to buy a gun that's already the caliber you want.
Regards
Bob Hunter
www.huntercustoms.com