Like the OP I swapped a fork into an OPC frame with large head tube. Because it was a "Velo Cheapo" contest bike I mixed the headset parts rather than buy new. I tried but could not get it adjusted properly. I assumed it was because the because the "diameters" of the bearing surfaces were significantly mismatched between the two donor sets. What SquidPuppet shared ^ looks like a much better way to go.