far back speakers in 01 fbody

thres the 2 sets of 6" and then in the very back, i have 3" bolt speakers, appear to be threeway...if i replace these should i get the same size thats in there or get 6x9's, thats what the cover/grill size appears to be...again, i dont know jack about car audio.

there is no 6x9 in your fbody. In the sail panel (right behind front seats), those are 6.5" subwoofers (bass). Unless you plan on doing some cutting just find a nice replacement sub to put in there. In the very back (behind the rear seat), those are 4" full range speakers.
There is a write up on LS1tech about upgrading the speakers in the f-body. Here is the link:

http://www.ls1tech.com/forums/forumdisplay.php?f=19