97 k1500 questions

ok i have 2 problems with my 97 k1500 (5.7) first off i ordered the wrong muffler (single in and dual out, instead of dual in dual out) from a guy on ebay its a 3" single inlet and 2.5 dual exit ( i forgot they changed the syle from single inlet to dual inlet in 96) can i buy one of these "Y" collectors ($40.00) and turn my dual pipes(inlet side) into single so it will work on my Flowmaster muffler ?? what size are the stock inlet pipes on the stock muffler 2.5" ?? ......second question sometimes maybe 4 times a month when my truck is cold and i put it into reverse and hold the brake it feels like something is "tugging" on the back of my truck but my truck doesnt move because i am holding the brake....i was wondering if this could be transmission or maybe rear end problem ?? thanks Jason

ok first of all that would be stupid buy the right muffler im shure you can get a 2 into one but keep it true duals everybody likes the way it sounds when sombody says its got true duals and im shure if people are going to look at it that you want it left true duals everybody looks at mine