The engines will be non-EFI with no reluctor ring or ability to sense crank speed.
There are many ways to determine crank speed. Before GM used crankshaft mounted reluctor rings they used the distributor. Early CCC vehicles were carbureted with a computer. The distributor provided an RPM signal. Some 305 equipped trucks in the '80s were carbureted. They used a knock sensor and a very simple system to retard timing to prevent knock. The distributor provided an RPM signal. There was no efi and the timing was controlled by vacuum and mechanical advance.

When the distributor provides an RPM signal in these vehicles the ignition module creates a 0 - 5V square wave out of the A/C signal created by the pickup coil. The ecm measures the time between each pulse then uses a number that represents the amount of crankshaft revoluton between pulses to calculate crankshaft RPM.

If an HEI distributor installed in your engine is retrofitted to use a module from a computer controlled engine then a 0-5V square wave signal would be available to use as an RPM signal in an add-on computer.