Converting to HEI, need 12v (Ign 12) source

That wire in earlier cars used to be simple..........came off the ign switch, went straight through the bulkhead and to the ballast

In the 74 book I show a splice where it branches off to the under--dash interlock box. Is it possible it got cut there?

I would access the ignition switch connector immediately and check there

Only place I see losing it is at the switch connector

at the branch splice to the interlock

or through the bulkhead connector