Crankcase exhaust ventilation questions

Hi All,
I have a 440 in an A body drag car with fenderwell headers. The crankcase is vented with exhaust evac via nipples in the 4" long bolt on collectors, and piped up to each valve cover. Works fine. I want to add a o2 sensor and was advised by the manufacture to have the sensor upstream of the evac nipples as the oil/water mixture being pulled from the crankcase may effect the sensor.
What I thought I would do is fab up some 30" or so collector extensions, move the evac nipples onto these about 10" from the end of these 30" extensions and put the sensors where the evac nipples were which is right at the end of the header. Someone suggested that the vacuum may not be as strong in this location, and that the oil/water that is pulled from the crankcase would effect the rear tires.
Any thoughts?