I have the 39mm PWK on my 87 Lt250r. 58 slow jet, 160 Main. Needle on second to lowest position. Perfect. My PWK has a sudco racing sticker on the side of the bowl, and the I.D. Measures 39.75mm with the calipers. I messed around with the Mikuni flat slides for a month. Took 2 days to jet the keihin.
I'll trade 2 34mm flat slide mikunis for your 39mm keihin.