Fan belt size

[cooky73]

I have recently changed to an alternator on my 1970 and tried using an 11.3 x 912 belt but it seems slack even with no shims in place?

Have tried a 10 x 905 as well and that is no better?

Can I get a smaller belt?

 

[67 BAY]

Had the same problem try a 10 x 888