ok, i just got my Hedman headers and Y pipe for my 84 TA, they didnt come with a O2 sensor, and i was wondering where would be the best place to get it welded in on the headers or the Y pipe?

On the drivers side header at the collector is the best place for the least amount of work.

The best place is on the y-pipe after the two pipes merge. But then you'll have to extend the wiring in addition to changing to a heated OČ sensor and adding the wiring needed for that.

what does the differance in placement do?

It's just the difference between taking OČ readings from one or both banks of cylinders.

wouldnt the placement mess with the ratio though? if i put it to far up, ill run too lean, and to far down to rich??...ill porbly just run it on the Y pipe right after the collector