I tried posting this twice on the Yahoo site, but it didn't post for some reason. I bought the Hi Perf coil from Summit. It's Summit's brand, 50,000 volt, 46,000 volts spark energy. I don't know what the difference is between voltage and spark energy voltage, but that's how they list it. I set my Plugs at .047. Works well for me. No Mis-fire at high RPM. As Tim said, I can't tell if it made any difference by itself, as I also put in the advance kit, and adjusted timing.
Here's a portion of the HEI article that is in the "Files" section of the Yahoo site. You should read the whole article if you haven't already. It's pretty good.........Don
Let's start with the coil. Its located on top of the distributor between the plug wire towers under a plastic cover. It's the "engine" that makes the sparks. Stock its capable of about 35,000 volts and so-so total spark energy. Its fine for a naturally aspirated street motor that rarely sees the high side of 5000 RPMs. It will provide the energy to jump a plug gap of .040-.045 with no problem in these applications. You can upgrade the coil with an Accell or MSD replacement coil that will jump the voltage up to about 42,000 volts and total spark energy will also jump about 10-15%. There are even hotter coils than this from Accell, MSD and others that will give you the same 42,000 volts but a LOT more total spark energy (like 50% more). Either one of these "super high output" coils is a worthwhile upgrade for a hot street motor- you'll get better response and HP especially at higher RPMs where the stock one hits a kind of "wall". A new coil is also a lot cheaper than a complete MSD ignition amplifier setup and at least 95% as effective at making HP in most naturally aspirated applications (the MSD will still give you slightly better mileage & emissions due to it's multi-strike spark capability below 3000 RPMs). With either hotter coil you can open the gap up to .045-.050 for just a smidge more HP. There are 2 different versions of the HEI coil and you need to make sure you get the right one. The only external difference is that one has red and *********** leads, the other has red and yellow power leads. You will need to know which one of these you have stock to order the appropriate aftermarket upgraded coil.