| Re: best spark plug gap?
Couple of things to consider.... stock replacement plugs were designed to work at a certain gap (.035" in this case). If you open them up much further the side electrode stops being parallel with the center electrode and you defeat any gains you get from openign the gap, plus the plugs wear out faster.
Now, there is probably some smidge of power and economy to be gained from a wider gap IF you use the right plugs and IF your ignition system packs enough of a wallop to reliably jump that gap for a meaningful period of time. Fortunately, even the stock computer-controlled HEI ignition system in good operating condition is perfectly capable of workign with a considerably wider plug gap, at least up to 5500 RPMs or so in a N/A motor- the HEI packs a heluva wollop at low and mid RPMs. You just need the right plugs for that much gap, which would be a set of Autolite #24 plugs (or equivalent) for stock cast iron heads. They're similar to stock but designed for a .045" gap. Up to about 5500 RPMs there's really no downside. I use them all the time with HEIs and factory cast iron heads of that vintage. They're one of my go-to plug choices.
Where the limits of the stock HEI system arise are in the upper RPMs. Like I said above, the stock HEI system packs a heluva wollop. But the fly in the ointment with the HEI is that it takes a LONG time for the coil to recharge to full power between firings. And as RPMs rise there is less and less time to recharge the coil between firings. When you get up over 5000 RPMs or so the coil is firing so often that it can't fully recharge between those firings. That's when the spark energy start to fall off quickly as RPMs rise. So if your combo is designed to still make good power over ~5500 RPMs or so, you might just start to run into those limitations of the HEI and lose horsepower in the upper RPMs. This is why guys who build motors that run up to 6500-7000 RPMs don't put stock HEI ignitions on them.
You might think you can solve this and get higher RPMs by just changing the coil to something better, since the coil is the "engine" that makes the sparks. And that's true to a certain extent- I have used the MSD coil in stock ignition systems many times with good results and it definitely has a faster recharge rate than the stock coil (it can recharge almost twice as fast). However, there's other limitations. One is that there is a trade-off between recharge time and total spark energy. The MSD coil actually has a WEAKER spark than a stock coil at low and mid RPMs, only surpassing it in the upper RPMs where the stock coil can't fully recharge fast enough! This is becuase the ignition module limits the maximum amoutn of current that can pass through the coil- you only have so much energy to work with and you can either make a heluva spark in a modest RPM range or a weaker one over a wider RPM range. All aftermarket coils play this game of trading maximum spark energy against recharge rate (some more successfully than others). There are other limitation inside the ignition module with how much dwell it can give in the upper RPMs that also affect things at high RPMs. Add those things up and you get the basic reason why guys with motors that make power at high RPMs don't run stock HEIs.
For a near stock TPI that doesn't often see the high side of 5500 RPMs the stock (or near stock) HEI ignition, in good working order, is a very well matched ignition system, however. | 
