I was curious as to why the link provided showed modifications to the case? I replaced ours, a 6300 Q series with a PD4645 and its taking out the old charger section and sliding in the new one. Transferring the wires. Its a direct replacement-no modifications to sheet metal needed.
The install you linked is older, on the Best converter page is a link to a more recent discussion by the same author.
Point is swapping out lower section fairly simple. Included 12v LED indicator fuse panel a plus, charge mode indicator so you can "see" (or change) what its doing. AND its quite...better charging.
http://www.bestconverter.com/4600-series-Upgrade-Kits_c_133.html .
There are several threads on the forum, what steered me to the Progressive upgrade. Ours also hummed- ran hot, but it was the fan that roared that pushed me to swap (and better charging).