Do you have this specific distributor?
http://www.summitracing.com/parts/nal-93440806
MAYBE it's curved for performance engines with large cams, and consistent with what I've said, it has an aggressive advance curve and MAYBE they are afraid the vacuum pod will give it too much too soon.
If you DON'T have this specific distributor, hook up the vacuum pod. You can always un-hook it. Or keep a piece of vacuum hose in the glove box and if it over-heats you can put it on. Check your mileage with it off, then with it on. Let me know if it causes problems, I highly doubt it. The following off GM instructions:
"Recommended distributor is the HEI (High Energy Ignition) distributor P/N 93440806. This distributor is a self-contained ignition system that includes a magnetic pickup, a module, a coil, a rotor, and a cap. The HEI’s large diameter cap minimizes arcing and cross-firing between adjacent spark plug terminals. The cap’s male terminals provide a reliable, positive connection for the spark plug leads. However, the HEI’s large diameter cap may interfere with other underhood components in vehicles not originally equipped with HEI ignition systems. Check for adequate clearance before installation. The HEI system requires a 12 volt power supply for proper operation. The HEI ignition system should be connected directly to the battery with 10 or 12 gauge wire through a high quality ignition switch. If you are installing an HEI ignition in an early-model vehicle originally equipped with a point-type ignition, be sure to remove or bypass the resistor in the wiring harness to ensure the HEI receives 12 volts continuously. Use distributor connector package P/N 12167658, which includes connectors and wires for the HEI’s tachometer and 12 volt terminals. Set spark timing at 32º before top dead center (BTDC) at 3000 RPM with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve".
Knowing the compression ratio, and the size of the camshaft, I still feel GM is out to lunch on leaving the vacuum pod dis-connected for a street driven car that will see extended light load cruise and needs to have good gas mileage.
IF IT WERE MINE, even if I DID have THAT SPECIFIC GM distributor, I'd hook up the vacuum pod and time it to 36' total and go from there.