Riddle me this, Batman!

Are you using an MSD amplifier?
My experience with those is that a non-MSD dial-back timing light does not work with those while the unit is multi-striking. Rev it up, to in the range of 3500 to 4000, after the the unit goes into Single-Strike mode, and check the timing there with the dial-back.
BTW
You can use #6 cylinder for timing adjustment, if it is more convenient, and that would rule out adjacent induction firing. #6 is exactly 360 degrees from #1, so it will read exactly the same as #1.
Also
>Sometimes I have had to turn my inductive clamp around on the wire.
> If you switch certain wires in the firing order, the engine may idle just fine, even rev up, allbeit with a larger throttle opening; 5 and 7 come to mind. 18436572 is the correct SBM order and #1 is on the driver's front, while #6 is on the passenger side and Third from the front ...... just in case you were a Ford guy..........