I've got the same problem....

My 50 248 powered 300 GMC had the narrow belt water pump. Well, when I fixed it, I put the "correct" one, but now I have to find a belt. I'm just reusing the old narrow belt, so the thing squeeks when you rev it up.....

Napa Web page actually has a search for parts.....and part numbers

1948 Dodge 2 ton
1950 GMC 1 1/2 ton
1952 GMC 3/4 ton