The original part number is 9433920 in Group 1.066. This is a 1/2 inch belt that is 57 1/2 inches long. The problem that you experience today, is that belts come only in 1 inch increments.
The common part numbers for a 57 inch belt are...
DAYCO Part # AP55 Gold Label Wrapped Industrial V-Belt; Outside Circumference: 57in; Top Width: .500in
Category: 12.7mm (1/2") Belt
DAYCO Part # AP56 Gold Label Wrapped Industrial V-Belt; Outside Circumference: 58in; Top Width: .500in
Category: 12.7mm (1/2") Belt
I would suggest going with the larger belt and seeing if you can get enough movement in the adjustment to make this belt tight. If not, then you are going to need the smaller belt, and it might be necessary to remove a part to get it on. I had this problem with my 1962 Chevrolet with the air conditioning belt. One size larger was too large, and one size smaller was too tight to get over the pulleys. I had to remove the air conditioning compressor, and mount the belt over the pulley, and then remount the air conditioning compressor. Once the belt had some wear on it, I was able to tighten it again, and all is well. You might be able to find another brand of belt that does come in the 1/2 inch increments, however, it might not be a wrapped belt like the one that I have shown above. These belts are available from
www.RockAuto.com at reasonable prices.
I would also suggest your measuring as Ed has suggested to verify that the information in the parts book is correct.
Here is the original post that Ed refers to my car...