The HEI probably won't help much with the power, but it will cut down on spark plug changes. The HEI coil has a minimum open-circuit voltage of 25,000 volts, and a good one will exceed 50,000. This makes sure the spark plugs fire every time, and they don't foul out as much as with a conventional ignition system. Also, switch to an AC "commercial" spark plug. They have a "C" prefix before the plug number, like a C45TS, for instance, instead of a 45TS. We use the commercial plugs in the race engines, and get more than twice the lifespan of regular plugs. Don't waste your money on platinum plugs, or other gimmicks! When you switch to the HEI distributor, have a good driveability specialist recurve the centrifugal advance to the same specifications as a medium-duty truck. It's probably set up for a passenger car now. If you've got the distributor number, I can probably tell you what it's supposed to fit, and what the current advance curve is. If you plan to do much towing with this rig, you probably need to install flat-top pistons and an RV-type camshaft for more midrange torque.
Edit: I just looked up the distributor specifications. The correct part # is 1110679 or 1110680, for a 1977 250 or 292 heavy-duty engine. Centrifugal advance should be 0 degrees at 550 distributor RPM (1,100 engine RPM), and max advance of 12 distributor degrees at 2,050 distributor RPM (4,100 engine). Distributor degrees are doubled, meaning your centrifugal advance will be 24 degrees over the initial timing, without vacuum advance. This is important! One degree over-advanced robs as much power as 5 degrees too late. I've proved this numerous times on the dyno. Vacuum advance on your distributor should start moving at 10" of vacuum, and it should get a maximum advance of 5 distributor degrees at 13 inches of vacuum. Vacuum advance goes away almost instantly when the engine comes under a load, so it's only for part-throttle cruise fuel economy. Good luck!
Jerry
