I know I'm an outlier as far as belt size, but it's worked for me for 14 years. I have Zena 150amp alts on my 120 Lehmans. I use a 5/8 Green V-Belt running on 2.75 pulleys on the alts. Can't remember the crankshaft pulley size and it's 700 miles away. At cruising speed of 1450rpms I get 120 amps out of each one. I've put about 4000 hrs on them and still have the original belts. I admit I do have some dust on the engine fronts. I redo the tension about every two years. The method I use for checking the tension is from the Zena literature.
"Static test belt tension can also be tested...by using a properly sized socket wrench on the power generator's pulley bolt to try to rotate the power generator in the direction of it's normal rotation for V-belt installations,and in the opposite direction for serpentine/flat belt installations. The pulley should be quite difficult to turn using moderate hand pressure. Often an engine will "turn over" before any belt slip will be noted. At the same time, you should notice a very slight slackening of the belt tension on one side of the pulley. If any slippage is noted, the point of failure should be easy to determine."