1202000470 is the part # for the small idler pulley next to the s/c. im guessing thats the 1 you need?

i wouldnt use the 6 rib pulley with the 8 rib belt it will prolley eat the belt or at least cause it to slip off.
as for the belt being 5 mm smaller thats quite a bit. when its installed look just behind the tensioner and you will see the slot were the tensioner rides. should be centered. to much either way may break the tensioner or the belt. this job is 10 times easier if you pull the fan out first. it takes seconds and gives tons more room to work in