[LH Lead-Foot]

Yes, ignition switch has an "S" terminal for start that goes to distributor +.
(It is cast into bake-a-lite plastic back on ignition sw.)
Does it backfire when you let off the key, or during cranking. If during cranking, the distributor has B+, but not on TDC #1 on power stroke. If backfires when you let off key, power to the distributor is coming for on and loaded with gas in cylinders.
Check the search box and check the box for "Forum" and wiring diagrams in color can be found. Many talented Tech's have posted their heart out as it take a lot of work.
I have never purchased an aftermarket harness, but most come with markings on wires. Just because I build my own, you may have extra wires that are not used depending on equipment, year and add-ons.
Let us know of shoot a PM for help.
Retired ASE Master Tech & Ex ACDelco instructor.