Check the strength of the spark with a spark tester. If the spark is blue it's strong, if it's yellow it's weak and something in the ignition system is on it's way out. By the way it sounds it's either losing or lost spark or it's lost fuel.
Gaps have very little effect on how it will run. You also have to keep in mind, the larger the gap, the harder the ignition has to work to arc the gap of the plug. Electricity is just like water, it's smart, but lazy. It will always take the path of least resistance so if there is a place it can arc to that's less resistance than the spark plug it will by all means do so.
If you've recently converted it over to a HEI check the voltage going to the distributor. A HEI MUST get a solid 12 volts to run properly. However, it can run on as low as 8 volts, but it'll run awful as a result.