I just went through that. I used the 3" pulley first and my engine would not cool. I switched to a 5" and got it cooling. The original for the '56 engine was 6" so if you could find one that size it would probably be better. (All sizes are approximate.)

Here's a link to the discussion we had.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.