One more thing,personally I think it's bad practice not to fuse the feed from the ignition switch to the HEI on an old vehicle you have invested time and money into.In case the wire gets shorted somehow,it'll save your wiring harness.My truck has a 20 amp circuit breaker feeding the ignition switch protecting the whole switch from shorts or overcurrent.I have no ignition switched items other than the HEI and a single small electric wiper motor.And in 10 years the circuit breaker has never tripped from the HEI load and or occasional wiper use.
Use a circuit breaker,the ones that plug into a fuse holder are only a few bucks.A 30 amp one will at least not nuisance trip and will trip from a dead short.
Here's some info off the internet,you be the judge if it's totally accurate,shows amp draw etc.
HEI current draw and rpm 
