Following up on this for those still interested.

I bought 24" NODrone helmholtz resonators from Synapseengineering [below]. Cut the ends off, used 45° bends and welded vacuum-actuated cutouts to the end.

When I'm not "on the throttle", they are shut and the resonators are working very well around 2,000rpm, where I had my issue. Moderate acceleration the cutouts open up and the car makes a little more noise.




Supporting info

J-tube Vendor
http://synapseengineering.com/synapseusa/products/nodrone.html?SID=76i1d7un3a55c1b9psfdog0eh7

Grassroots thread
https://grassrootsmotorsports.com/f...tor-or-helmholtz-resonator/129212/page1/

Calculation details
https://www.performancetrucks.net/f...t-resonator-tube-eliminate-drone-489463/

Vacuum cutouts
https://proflowexhaust.com/products/proflow-vacuum-actuated-exhaust-cutout-valve