Well...
As a starting point, put the effective center of the tank in line with the outlet of the spray bar, then adjust from there.
You may just have to get things as close as you can, and live with it not being ideal.
I hesitate to give this advise because I never did really get the hang of metal tanks, but here goes: since in a uniflow tank the effective center of the tank is the location of the vent, make a tank that comes to a wedge at the very top of the tank, and put the pickup and vent there. Or, if you don't need that much height, put the uniflow wherever it needs to be, and the wedge to match. This should, at the expense of losing some volume, raise the effective height of the tank. Obviously, you'll have the joy of building your own tank.
You could also just take a regular tank and move the uniflow up. This'll mean that when the tank gets down to the dregs it'll lean out more in level flight than inverted -- but you'll be able to just modify an existing tank instead of having to build up a new one.
Or just live with the fact that the tank height won't match in level and inverted flight.