[VetteVet]

Using a little common logic. If your engine turns over then the block is grounded because the starter will not work unless the block is grounded.
There is a gasket between the base of the distributor and the block but The distributor gear is engaged with the cam gear and the distributor hold down down clamp is bolted to the distributor and the block. So the distributor is pretty well grounded.

If you tested your HEI wire from the fuse block with a voltmeter and you put positive on the wire and the negative to the block, the block would have to be grounded to get a reading.

I test my distributors by holding them in my hand and running a ground from the distributor body to the battery negative and a positive jumper from the battery positive to the distributor BAT terminal, then I just spin the gear by hand and the spark will jump from the towers to the distributor body. Try it with one of your three or all three. Make sure the plug wires are off and the rotor and cap is in place.

You should be able to just use the wire from theIgn Unfused and a jumper from your battery ground on one of the distributors.

I'm assuming that your rotor is turning in the distributor and the cap is on correctly.