I used 170mm cranks for many years. When I started to ride mountain bikes, too late in life, the cranks were 175mm. I do think I can spin the 170’s easier than 175’s.

I found an NOS FC-6503 for a good price. They are 172.5mm. At first I thought they may not have been a good choice, but 8 years and counting, they have been fine and I don’t even notice any difference.

While not big proponent of KOPS, I tend to error on further back than too far forward. For me that seems to have worked out for my hips and knees over the years, than the 5mm difference in crank arm length.

John