Chuck, this seems to be as much a product of the carrier as the lens. On mine (with the square bottom bellows) the negatrans tends to vignette on the right top and bottom when there is not enough clearance to the top, and on the bottom with the glass carrier. Not much, but this is where it will first show up with a 135mm Nikkor. No problems at all with the 150mm. Optically, the negative is centered.
There is only a slight difference between the condensor and diffuser head diameters (6 3/8 versus 6 1/4" respectively) The thickness of the carrier can play a role also. If you are working toward full-frame, I might suggest trying an 150mm and seeing if that solves your issue.