When I was young and built hot rods the only timing light we had was one that ran the Xenon bulb directly off of the coil voltage and went inline with the plug wire (no induction coil).
I tried to find some info for you but the internet has very little info about these old things. I did manage to find a single picture of one.
I do believe they cause a little reduction in the spark on #1 but it is minor and should easily be accounted for. One of these should be easy to make. Connect the bulb between your distributor and your plug and you are done.
EDIT: looks like you can still get them in a cheap tune up kit https://www.motomobil.com/engine-tester-kit-incl-strobe-timing-light-dwell-angle-teste,3237,49610130-1.html