Tommy;

AFR Sensor sniffs exhaust and needs to be hot, so should be as close to the combustion chamber as installation constraints will allow...and that will mean one bung per cylinder only a few inches from the cyl head, and that you change sensor location during tuning (or fit 4 sensors and a switch if you're rich)...sensor located at the collector will obviously only see an average as the exh gasses mix, so is only marginally useful for tuning...

Sensor (or2) at the collector will also be sniffing cooler Exh stream so reading would be diff from just off cyl head. Characterization of the readings is necessary. With 4 bungs, after tuning operations, you can leave sensor in the location of your choice, and plug the unused bungs.

I advise edumacating yourself on AFR sensing and metering before haphazardly cutting metal...there's a lot to know, and more to get right and wrong...plus interpreting your AFR meter readings! I'll see if I can relocate a very good article published for the General Aviation / Homebuilt Aircraft constructors which had much good applicable info...

See also: https://www.sw-em.com/Carburetor_Additional.htm#Wideband_O2_Sensors ...not much detail yet, but I'm working on it...!

Cheers