A flat fingerboard exaggerates some of the shortcomings of a staggered single coil with modern strings. So on every flatter than 7.25 board the g string stands out like sore thump imo. My recommendation is to use a ssl-6 with a flat fingerboard unless you use a wound g string.
I am with you on the glorious antiqity neck with an A3. I had an brobucker in the bridge position over 10 years ago. I did not gel with the brobucker, even with an A2 or a A4. Sorry, never tried an A3. The brobucker was too dark and wooly for me. Now my bridge is the AntB with an unoriented A5.
Not Duncan, but should fit.
https://www.philadelphialuthiertools.com/pickup-making-parts/pickup-covers/trembucker-humbucker-pickup-cover/?searchid=2014748&search_query=trembucker+cover
You have to look at the amp first and listen to it. Most probable it uses a cap can, which are some caps in one housing. Then you should where to buy that specific cap can. If they are out of production, you need to look for a suitable substitute (little higher or lower caps with a sufficuent...