Correct arm and index finger for ECG?

I do not think it matters. There is an Apple Watch -> Settings -> General -> Orientation for people that wear their watch on either arm.

You just have to use the index finger from the other hand on the Digital Crown.

I just switched my Apple Watch Series 4 to my Right Hand, fixed up the Orientation information, then took an ECG, with my left index finger on the Digital Crown of my Apple Watch sitting on my Right wrist.

Set your watch to be worn on the left arm. Use your right index finger for the ECG. The index finger is only providing a conduit for the electrical pulse of your heart to reach the  Watch.

Having an arterial blockage should not affect the ECG app.

You're welcome.

To clarify, the fact that you have an arterial blockage should not affect the ECG which detects electricity, not blood flow.

Best of luck!