Re: Adding a Front Brake...advice, suggestions, please
The major issue I see is that the caliper brake you are using attaches to the steering post which does NOT pivot with your springer fork suspension while your wheel DOES pivot. This means that though you may have the pads lined up correctly when you are off the bike, even the act of mounting the bike puts them at a slight angle from perfect alignment with the rim. Stopping only makes this worse as it moves the center of mass foward causing the bike to "dive" and changing this angle even more.
You can get around this either by using the drum brakes mentioned or replacing your springer fork with one that has v-brake bosses on it and then installing v-brakes.
I had a similar situation with my military bike but went the v-brake route and vastly improved the braking performance.
If you ever mount fenders on that bike, make sure the front fender mounts to a bracket spanning the front struts on the springer and NOT to the same hole your current brakes use. I did that and the flexing of the fender caused it to break in half and roll forward on the frame, locking the front tire from moving. Luckily for me, this happened when I was off the bike rolling it down the stairs. Had it happened on the 8 mile ride I'd just finished someone would have been scraping me off the street.