While not the Hemi unit, this RB manifold is similar with the removable lid. The 6bbl application needed the front dam added and the rear divider removed.

I’m thinking the Indy Wedge Modman got its design influenced by the STR.

As for backfiring, I would make sure you have a separate ignition on source.
Spin the motor then hit the ignition. This solved the backfiring on all the cars we ran with the STR and RR.