I'd planned for another visit to the circuit, but during inspection/oil change...found a hole where an exhaust manifold stud/nut was supposed to be!
No doubt due to my lack of torque wrench use in this challenging area...but the stud too?
3 weeks later...Chevy in Kansas City delivered:
5 minute install...back in business.
Friday is an off day this week...maybe with some luck might break away...