I never used a uniseal... and I don't think I would in this situation.
I think you can fit a bulkhead per tank and a union in-between in a 7" spacing.
As for alignment of the holes, make a jig out of plywood and your holes should be dead on. The recommended size hole for any bulkhead always allows for some adjustments. In the end for a large error you could always adjust one side with a thin material under one tank.