I don't know what are the "best" widths, but it will look best if the front axle and rear end are close to the same width. In my coupe I used a Ford 8" rear from a Maverick and the front axle from a '60 Jeep station wagon. They are the same width. My son used a Maverick rear and an Econoline front axle in his A sedan because they are the same width and have the same bolt pattern for the wheels.

As far as your frame is concerned... I partially boxed a Model-A frame once. The next time I undertook a similar project, I found it was quicker and easier to just fabricate an entire new frame from 2" x 3" tubing...