Chrysler already did that work for us. The modifications are diagrammed out in every Direct Connection Engine book (the result of hundreds of dyno pulls). At the suggestion of others, I had these mods made before I installed the STR-12 on my Dart 340 and have had nothing but good times out of it. That said, the cast tops are too weak. The original on mine was repaired three times. I found a billet one at the Mopar nats 10+ years ago - much stronger. The lid is reasonably simple so tracing it out with a CNC program and cranking out lids that solve that problem would be a breeze.
What I don't get is that Edelbrock reproduced a run of the STR manifolds for AMC 390 a few years ago. Why not an STR-12? I think that an AMC group got paid pre-orders for most of the run. I can't believe there isn't more demand for a sbm version than the AMC engines.