[DonC]

The best place to locate the sensor is within about 18" of the exhaust ports. The problem with our cars is that the collector is usually located much farther away than that. That plus the fact that the sensor should be located within 90 degrees either side of vertical compounds the problem.
I'm running the sensors for my Mass Flo system in a single tube on each side and get reasonable results. Quite a ways out from ideal but you end up having to work with what you've got if you want to go with an EFI installation.
Another way to go would be to build shorty tubes into a collector closer to the engine and then 4 faux tubes running out of the body to a faux collector in the normal location. An awful lot of work and not sure it would function any better.
I'd say go with mounting in the single tube and see how it works. If you end up having to go to the collector later you can just use a threaded plug in the single tube bung. If you have to go to the collector you may be able to locate the sensor on the back side of the collector and port the body to allow the top of the sensor and the wires to pass through.
No body said these cars were easy.
DonC