Spindle length is determined by the specific crankset you are using, but you need to start from the back and determine the chainline, so that you can match it on the front. There are too many combos to easily know which spindle length will fit, especially with an older crankset. You may need to dry fit a spindle and your cranks to see what is needed.

Frankly you are better off buying a crank and spindle together. Building a bike from scratch is either a headache or a wallet drain - or both. It's a very inefficient way to get on the road or to learn a lot about bike maintenance.