I've had two pairs of cranks re-drilled down to 150mm from 170mm, by Highpath and they did a good job. Cranks need to be solid and not fluted.
I also bought two pairs of SINZ cranks - lengths 145 and 150mm. These are square taper cranks and are very solid. Unfortunately SINZ cranks have mostly been discontinued. But you can find some on ebay.