You can't power the HEI with the same lead the points distributor used because there should be a ballast resistor that steps down the voltage.
A relay setup is best bet. Pull power off horn relay as mentioned. Trigger it with the factory IGN lead - it might not be hot in start position, test it. But there should be another lead top the coil that is hot only at start.
Here, ignore the details on this pic, just look at the circuits.
87: to the HEI - 12g is more than sufficient. W/O looking I'd bet 14 is ok.
30: with a 10 amp blade fuse to the horn relay/distribution point. Again 12g/same as 87
85: ground - small is fine, 18g or whatever
86: Hot@run (IGN-pink?) and hot@start (not sure color)
