Mini Starter and Neutral Safety Switch??

I dropped an A Body drivetrain (318/904) into a 1939 Dodge Truck I am building. I am installing an American Auto Wire Universal Wiring Harness.

My question is do I need to run a control relay if I am using a Mini Starter with built in Solenoid? The wiring diagram for the harness I am installing suggest that I run directly from the Ignition switch to neutral safety switch and the back to the Solenoid?

My concern is the harness is using 10 gauge wire provided for the ignition switch to the neutral safety switch. The neutral safety switch harness on the 904 is 18 gauge wire at best. Is that 18 gauge wire sufficient to run back to the solenoid our should I use an additional relay? How much amperage does a starter solenoid pull?