[njsteve]

Those cars are very sensitive to improper grounding. If you don't have the harnesses and the firewall to engine and radiator support to fenders grounded properly, the voltage will back feed into your harness and find another way to ground. That other way can be through other live circuits.

I found this out the hard way when I ended up frying my distributor points and block temp sender because I didn't have the engine to firewall, radiator support to subframe and engine to frame grounds hooked up.

You did mention that it is the neighboring fuse that is blowing so it might be feeding back through the nearest fused circuit. Check to see that you have your front fenders/radiator support grounded properly, and the dash ground wires hooked up.