OK, I edited your post to make it bit easier to follow..hope that's OK by you..
First, It sounds like your charging and electrical system is functioning fine..and the battery is up to snuff..It may have been a fluke before , something sticking on..running the battery dead..as you said, watch it for a few days and see where it goes..
Something to check, the belt may be "Just" loose enough not to charge under Peak loads .. recheck that and tighten a bit if you think it needs it.
Now about the Instruments, You may not have enough of a load on the dropping resistor to effect enough of a voltage change on the instruments..
A regulator does not depend on the output load to drop voltage..it's 12 volts in a chip, and 5 or 6 volts out , regardless of if it has anything hooked to it or not consuming power..
You can get an instrument regulator at any auto parts store for Dodge or Chrysler in the mid 60's to 80's range of years..If you can't find one I can tell you how to build one from a hand full of parts you can get at rat shak..consists of a 3 terminal regulator chip, a heat-sink, some 18 gauge wire and a bit of your time..wire it power into the chip, ground , and 5 volts regulated out..The only rule is you must keep the draw UNDER 1 amp..(instruments won't draw that much..)
Just had another post on here SAME issue..the guy built one , installed it and was simply amazed how well they worked after all these years..
This is one of the "Little headaches " that crop up on "split buss" systems..(Why I do not like splitting a buss..6 and 12..and advocate all or nothing at all..)