I have an 80mm at the moment but am getting lower back ache. Will sliding my seat forwards and adding 10mm to the stem length help this?

There are so many variables here that it is impossible to give an straight answer. Is your seating position uncomfortable? Have you tried lowering or raising your bars. Hunched or Stretched out? Do you get back ache in the first 10 minutes or is at towards the end of a 3 hour ride etc?

There are so many issues with your back, it would be better to see a professional than take internet advice. I would have thought that pushing your saddle forward and fitting a longer stem will place more weight over the front of the bike and pressure on your arms/shoulders. It could be a solution but I am doubtful.

Jay

What he said + try stuff and see if it works.