>>Does that ballast wire go right back to the fuse box<<
How many ways can I say NO.
The ignition circuit never went through the fuse box. A 12 gauge wire goes directly from the ignition switch down to the inside of the hulk head connector. On the engine side of the bulk head connector it connects to the ballast wire.
The ballast wire goes down along your 6 cyl valve cover where it was crimped together with two other wires. One wire is usually yellow and it goes down to the Solenoid R terminal.
For an HEI ignition circuit this wire should be eliminated and no connection should be made to the R terminal.
The other wire crimped to the ballast wire was 14 gauge wire that then connected to the HEI.
This thread may help you.
https://www.67-72chevytrucks.com/vbo...d.php?t=708975