Just a guess?
The exhaust restriction where the valve head comes too close to the back wall has been explored courtesy of Curt B. This is fixable in part without spending lots of $$, and IMHO required for any performance use (along with bowl enlargement, multi-angle seat, small radius where margin meets the valve's seat, back-cut valve at 30°).
The intake/exhaust flow bias is also affected by rocker ratio, especially the intake - but there's nothing available for the 235 except the odd museum piece.
