Install considerations for your 02 sensor should be heat of sample gas. You will have great results if you keep it close as possible to heads like it is now. You mentioned welding above the collector and on the shorty that is pretty close to the head flange.
Mounting 1 sensor in the collector where 4 pipes merge into common pipe will give you an average sample of that bank of cyl's and the EGT will still be high. On a single sensor install you have choosen great location. If you mounted downstream or in the crossover pipe between new dual system it would seem to be in a better place from an average of sampling but would give very different reading because of extreme heat difference.
In a system with multiple sensors ( in E manifold = before the Cat = after the Cat) Design accepts lower temps but now compares to each other for funtion. Said different if now a system in manifold try to keep in close to same range. If changing to multiple sensor ECM then lower temps are OK and duplicate their locations as close as possible. BEEN THERE and found this out the hard way!! Without changes to system I read .82 in collector at end near muffler, and .71 some 10 inches closer to eng. and .64 in #8 tube at merge to collector on this Toy