Diagnosing poorly running Mikuni setup. Not the usual causes

Excellent. There is one exact sweet spot where I mounted it on top that was both flat and that cleared the stock fuel sender arm movement.  

If you do get in a bind so to speak with the sender arm, there is a great alternative sender sensor that I discovered while working on the 510 last year that you could install at the same time as the pump.  

Check this baby out!  http://shop.classicinstruments.com/sn38t-3  They come in various lengths, and the ohm range for this one is darn near perfect for a stock gauge.