[Signman]

It probably is better on a carburetor car to have the O2 in the collector if only one. Intake manifold can make fuel distribution uneven cylinder to cylinder. If in the collector you need to have an extension with the O2 as close to the convergence of the four primary as possible. One in each primary within 20 inches of the exhaust valve is best. In fuel injected cars because fuel distribution is pretty equal based on the location of injectors one in primary tube can work well. It does for me.

Understand that everyone has their own opinion /experience and different O2 locations work good or not so with varying assemblies. What is important is that what ever data reviewed is understood for what it really is.

O2 in the primary tube will see less fresh air than when installed in the collector. Am convinced with the cams we run O2 will see some fresh air no matter where located and more so in the collector.

In the collector AF will read more lean than if in primary tube. Check plugs and exhaust ports / headers. They don't lie. Be careful when helpful people say it should be this or that AF. My last motor was lifting the head trying to run it as lean as was being advised. They had O2 in collector while mine was in primary.

If you would like to get in the ballpark quickly spend a day on a chassis dyno with a good tuner. Review sheets from your the best runs. Those AF numbers are real for your assembly. Maintain and test variations on those.

In Div 1 there is a good tuner in Danbury CT.

Good Luck and Have Fun!