It is quite a big frame, the seat tube is 60cm centre to top and I think the top tube is 57cm, I am 6ft 2 and having 34 inch inside leg measurement in theory it is about the correct size.
I'm 6' 0" and ride a 58c-c (about 60 c-t, I think) with a 57 top tube it's about right for me and I've got long legs.
I think Nob's got a point in saying it might be too small for you.
A larger frame and shorter stem combined with a saddle like the San Marco Concor Supercorsa might go a long way in relieving the stress on your lower back. You could also try and push the saddle forward, that helped my back, I'm upgrading all my saddles to the Supercorsa as it's amazingly comfortable and kicks up a bit at the back which supports your bum on the seat.