there was a flat non washer pal nut holding the assemblies together,Similar to the one on top of the front shocks - then a captive conical washer nut holding the assy to the body visible from inside the trunk.
both are 10-24 machine thread ,..Bad news is I've never been able to locate a source for the correct flat palnut. AND the 'almost' correct conical washer nut has been discontinued from the manufacturer.
This is the closest one that SHOULD be on a 70 Challenger ( but should be yellow zinc di-chromate plated ) now no longer avail. AFAIK