Another pic from the SM that's relevant:
I think at least it should be possible (perhaps even superior) to block off the 4x outlets labeled B (2x heater core), D (throttle), A (says "EVAP piping", but it's the other side of the throttle stuff, I traced it myself a long time ago).
You could truly seal off (like, weld shut) the two extraneous nipples on that rear water outlet assembly (the ones with hose clamps numbered 6 and 1). Then on the driver's side you could permanently plug the upper hole of where Gasket #15 is. It's the bottom hole in gasket 15 taking in bypass water from that #19 bypass pipe that seems like it could still be necessary, maybe...