Just checked the Osborne Engine Assembly Manual for 1966. Page 48, Power steering drive Std and Extra cooling engine-289, Part number
C6DE-8620-A or C5AE-8620-U, both are 1/2 inch wide by 45.5 inches long. Are you sure you have the routing correct? The belt only goes over the forward crankshaft pulley and the power steering pump pulley.
I bought The same belt for my car, (power steering, no AC, 289) from Kevin Marti and it fits perfect.
John
Just checked the belt on my car. It's marked C5AE-8620-U