I don't know if it is the MSD or not, mine is wired per the instructions, did it too many years ago (at least 10 years) to remember how.....
At the time, I also changed my dash panel/gauges (65 Mustang fastback) from the factory panel to a custom made panel with analog gauges, also added a JVC radio with a clock and the memeory thingy for the radio settings, that's when it would run the battery down, takes at least 2 weeks......
I chased down some stuff and found one thing and fixed it, but it kept doing it, so it is too easy just to pull the positive cable off the battery, so, being a lazy fellow, that's what I started doing and since being in the habit, it's no problem for me, but still didn't fix the problem, and after 10 years, it'll probably never get fixed correctly by ME............
David