I am converting my mountain bike into a comfort bike. Everything else is in place and working nicely but the crank. Now that I am not climbing steep hills, I need more gear ratio. When I stand and mash the pedals, I go so fast that the ratio becomes a limiting factor.

With that in mind, I want to change out the crank. Today I have a 22/32/42 175mm Shimano octalink (FC-M341-8).

First question is, Shimano has cranks labeled like so 'FC-M341-8' and 'FC-M341'. I assume the "-8" means 8 speed, but what is the one without the -8? Is that a 9/10 speed or what? The FC-M341 comes in 28/38/48 or 22/32/42 if that helps. ( the -8 only comes in 22/32/42)

Second question is, my current crankset is 175mm. I see a lot of 170mm cranksets. What is the significance of this length? Which should I prefer? I figured a MTB would take a smaller one, but since I won't be hitting the dirt anymore, a larger one should be fine. So going smaller at this point wouldn't make sense. But that's just my guess.