I just used a piece of string around the pulleys then measured it. went to parts store and tell them what size. May take a little trial and error because different brand belts can have thicker Vs than others and ride too high in the grooves. one of those emergency adjustable fan belts will work too. Put it on, adjust to right size and take it down to the store. They'll have a device to measure it there and match it up.
It's not the leaks that bother me, it's when they stop that makes me worried.
1988 Waggy SOA stretched, cut, Chevy R. 4" springs in front, shackle reversed, stock springs reversed in back, Edelbrock manifold and carb, mini-spooled front, lunchbox rear, currently on spaced 35s.