Somn,
You will see there is not much room to slow down, about 20kts would be it.
This is at F370 which is lower than usual for an NG. At higher levels the margin is much smaller, so your 30kt reduction is not going to happen.
I know you said it was a hyperthetical situation, but it didn't take long for blokes (Capn Bloggs) to start calling people d!ckheads, and assuming who the operators would be.
As for a 737 overtaking a 767 by a large margin (and only 2000' higher) - that will never happen unless the 76 is on a big speed reduction.