Your fuse block will have two male spade fittings on the front side. One is for keyed-power, and the other is constant power. You may have to unbolt the fuse block from the pillar to see the fittings. :
Another suggestion would be to use your existing ignition-on wire in the stock loom and instead of running it straight to your HEI (it's too small) run it to a 30 amp relay that draws it's power straight from the battery. Then run a 10ga wire from the relay to the HEI. The HEI calls for a 10 ga wire. However I did wire mine direct, with no problems....yet. But I plan on wiring in the relay.
I'm still a "Rookie" so somebody please grade my paper. Seriously
Like I said, no problems with it wired direct, yet, but my experience with automotive wiring is, the bigger the better, with the appropriate fuse and/or relay. I don't think it is as much a performance issue as it is a safety issue. If GM calls for a 10ga wire, believe me they're not going to specify a wire any larger then necessary. IMHO
I hate electrical fires, they're so very....well...stinky and they fry a lot of sh*t