It would be very difficult to safely jumper a GM starter solenoid with a screwdriver, especially with headers.

Does your battery cable (thick lead) run directly to the starter, or possibly to a remote solenoid on the firewall or fenderwell? With a remote solenoid, it's easy to jumper. If not, you can buy a simple remote starter switch which connects from the fat cable to the S terminal for remote cranking.

If you have wipers and cigar lighter, you've established that you have primary electrical. If you can buy, beg, borrow or build a remote starter switch, you can test your starter solenoid and ignition switch easily.