Headers vs 340 manifolds

As to closing the exhaust, we ran "caps" that used the 3 bolts and a plate to close the header, then ran a street exhaust from a tube welded into the inboard side of the collector to the exhaust pipe ahead of the muffler.
Just unbolt the cap, and go racing.

Cheers
And nowadays you can buy them premade with a remote control operated valve that activates the cutout, how times have changed eh?
https://www.amazon.ca/Energy-Exhaust-Cutout-Electric-Wireless/dp/B01MXLXT0G/?tag=fabo03-20