I am looking for some guidance and input prior to spending any money.

I am 6'2", with long legs. I also have a bad back.

Yesterday on my century my back was completely in knots. Could this be due to a top tube that is stretching me out too far? I have adjusted the saddle so I am set up comfortable, and I have shortened the stem.

I was on my Lemond Nevada City...61cm ST and a 60.5 cm TT. The bike is comfortable and nice riding...but I am wondering if my Bianchi Limited would be better for my back? It has a 62 cm ST and a 56 cm TT.