Hi Dave,
Here are some notes on Ford CMP sidevalve V8 fan belts that I compiled a while back. May be of some help. I have Total Drive Plus B52 belts on my engine.
Fan belts: Ford originals marked Ford 51-8520, 55-5/8 OD, 5/8 wide at top (actually 21/32)
New ones: Total Drive Plus, Match Free* B52.
Correct one- has outer circumference of 52 + 3 = 55
Understanding the Part Number
B The B stands for a B section v-belt. This is considered a Classic v-belt style that has a top width of 21/32 (0.656) inches, but is commonly referred to as a 5/8 (0.625) inch belt. The thickness of the belt is 13/32 (0.406) inches.
52 52 inches is the inside circumference of the belt. For B-section v-belts, the outside circumference is found by adding 3 inches to the inside length of the belt. So, the outside circumference of the B52 belt is 55 inches (52+3).
*Match free- means the belts are manufactured to close length tolerances eliminating the need for matching the belts on multiple row pulleys such as the double row pulleys on the CMP truck engines.
Here is the link from where I obtained my info.
http://www.biedlers-belts.com/gates-b52-belt.html
Cheers,