The pulleys for the narrow fan belts (3/8") were used from 1953 onwards on 216/235/261 engines.
On truck engines from 1954 onwards, the diameter was 7.25".
It looks like a 3/8" groove 6 1/8" (smaller diameter) pulley was used on 1953 car & truck engines.
There were some 7.5" double groove pulleys for cars (1953 and later) - they would e close enough in diameter.
From 1937-52 the 216 harmonic balancer was a little over 6" in diameter.
The 1941-1952 235 harmonic balancer (5/8"groove) was 7.25" in diameter.
Is that sufficiently complicated and confusing? 
