I had 18" WM3 Borranis front & rear. 110 tyre on the back & 100 on the front. The handling at high speed was crap. Also, dropping the 19" mudguard down didn't really look the proper part as it was a slightly larger radius.
I now have a 19" WM2, 100X90 tyre front & my old Borrani 18" WM3, 110X90 tyre on the back, properly centralised & an ISO head steady. The handling is fantastic. I would really recomend the Metzeler Lazertec on the back, I think it is a wonderful tyre.
If you speak to Norman White, he will go for 18" front & back, 100X90 front & 120X80 back, BT45's, but would prefer a WM4 on the back. He can also build your wheels for you, he did a good job on mine & you can also discuss things with him.