the numbers on my belt for the champ billet bracket , a/c and tensioner delete looks like 50510 , possibly 60510 , either way its the 510 thats important 51" , thats with stock crank pulley and alternator with short rear brace .
if you have undersiize crank pulley or a non stock alt pulley or you have the alternator mounted with the long rear brace you will need to tie a string over the pulleys and then measure it
if you get 48" thats a 0480 belt , if its 48 1/2" thats a 0485 belt and so on