I believe you could do that, Robert. The modern approach (at least from some makers) is to use a few different relays (usually found in the fuse box) to activate different items. I think the thought behind that is twofold: not to overstep the ability of a relay and by keeping the amperage across it lower not to shorten the life of the contact points in it. The extra draw isn't voltage but current (amperes). For years I've told folks to think of voltage as the size of your arm and amperage as the strength of your arm. My original comparison was "would you rather arm wrestle with Mahalia Jackson or Arnold Schwarzenegger?" Their arms at the time were about the same size.
To your question, yes...30/40 amp relays for the HEI are common and popular. They're cheap and dependable. Also if you have a high amp electric fan, most of those will work with the same relay. The draw from an electric fuel pump, reverse lights and the overdrive solenoid won't be too much, really. With just what you've given us I would say you could come off the terminal block with a 30/40 relay and dedicate it to the HEI. Here's the thing: your HEI at start up might draw 25 amps or so...maybe only 20 but that's still a good bit. If you have a 16 inch electric fan, it might take another 25 amps to run. We're already to 50 amps. Even if your fan is thermostatically controlled, if your engine is shut off at operating temp and the temp increases, the fan and the HEI will operate at the same time. At that point you will be right on the edge with 50 amps. So, to hark back to my second sentence above, I'm generally more comfortable with proper size wiring and separate relays. Hope this helps.
