The way I test for this is as follows:
Rig a meter probe as close as you can get to the blue VR power (IGN) terminal. Don't unhook anything because you want the load (current draw) to be "as run." One place is to probe the "key" side of the ballast, the other is the blue alternator field
Hook your remaining probe to battery + and turn the key to "run" with engine stopped. You are hoping for a very low reading, the lower the better, and less than a few tenths of a volt
If your read more than .3--.4V (3/10 of one volt) that drop will be ADDED to the VR set point when running
EG let's say you read 1.4V. You run the car and if the VR is charging nominally 14V, the battery will read 15.4V
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As Greg said above, "one way" out of this is to clip the blue ignition feed wire coming out of the bulkhead. Use the end at the bulkhead to trigger a relay, and get a breaker or fuse and feed the relay contacts off the starter relay stud. Now connect your other end of your clipped blue wire (IGN run or IGN1) to the load contact on the relay
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You also want to check the GROUND end of this. Do this test first with all loads off, and again with headlights, heater, running
Run the engine, set RPM to simulate low to medium cruise RPM. Stab one probe into the battery NEG post, and the other into the VR mounting flange. Be sure to stab through rust, paint, etc
As in the first test, you are hoping for a very low reading, zero is perfect
IT DOES NOT HURT to improve the battery (block) to body ground. What I do on a V8 is bolt a short starter cable to the rear of the driver side head. Look at your pass side head front, the same bolt holes are on the driver side rear. Bolt the other end to a good through bolt in the firewall, or to one of the master cylinder studs.