The 170 and 175 are the lengths of the crank arms. I would stick with the length you have if you have no issues with your pedal strokes. Google up crank length and you will find it is often just a user preference with some thinking it is a vital, crucial detail and others not noticing any differences. I just checked two of my mountain bikes I have in the house now, and the road bike, and they are all 175. I am 6’4", and some think taller means longer cranks. I am in the boat of not having a preference. It seems like larger bikes come with 175’s, though my sample size of three bikes is hardly statistically sound. Look at the inside of your cranks, near the bolt hole for the pedals, I bet you will see a 175 or 170 stamped in the crank arm.
There are two different BB widths, but I do not think the cranks are specific to either, they should fit no matter what the BB size. It would be the crankbolt lengths, not the cranks themselves, that would be the culprit if anything is specific to either BB size.
When you say splines on the left crankarm are stripped, are you talking about the bolt and its threads that goes through the crank into the BB, or the threads in the BB itself? If it is just the bolt, then that is a cheap fix. Bolts are sold separately. If it the threads within the BB, then you would likely need a new BB as well.