I want to end up using a 120Vac source and a step-down transformer to tie into the full-wave rectifier.
That will be the correct way to use the diode bridge, instead of the way it is shown in your schematic after a battery.
Your life will be easier if you start out with a diode in the place where you put the lower mosfet. (At least during these early development steps anyway.)
Right now you might want to experiment with an interactive animated simulation of a buck converter.
It will show you how coil action takes place in two stages. It will portray how current flows alternately through the mosfet, then through the diode.
Click the link below. It will open the website
www.falstad.com/circuit.
It will load the schematic and run it on your computer. (Click Allow to load the Java applet.)
Click the switch to close it, turning on the mosfet. Then let up the switch.
As you will see, there is no particular voltage being applied to the battery. (I used 24 V in this case, but it could be any of the figures you mention). There are only pulses of current going to the battery. You must install the control circuitry which will limit the intensity of the pulses.
The simulated flow goes up to 18 Amps. I suppose the limit is due to a parameter in the mosfet model. With the right component you will be able to obtain 40 Amps as you planned.
Below is a screenshot only. It is not animated, nor will it interact.