I have the same model bike as yours, and I've also had charging problem. It would start missing, then die in the middle of a ride. I think I am finally over it (after replacing an AGM battery that I killed too!) Here's what I did:
I went with a new harness from Parts 'N More.
http://www.partsnmore.com/parts/honda/cb750k/?filters[category]=electrical&filters[fitting]=custom
I still had problems after this, due to the poor condition of the connectors on the rest of the bike. I tried my best to clean all the female terminals, but the corrosion was so bad on the connection from the alternator that it overheated and melted it, as you can see here.
http://forums.sohc4.net/index.php?topic=90323.msg1010735#msg1010735 . I kludged something together with oversized bullet connectors and electrical tape to get me going, and eventually replaced the OEM style plastic connector with one from Oregon Motorcycle Parts.
http://www.oregonmotorcycleparts.com/connectors.htmlSince I was already getting something from OMP, I bought a few packs of new terminals and replaced all of the corroded ends on connections from the rectifier, alternator, ignition switch, and fuse box. I also picked up an ATO/ATC style fuse box. Now, the system works beautifully. Headlight seems brighter, engine feels stronger, and my breakfast even tastes better.
Before you spend a bunch of money on a new reg/rec, do the tests as found in the FAQ section. They might be fine.