My truck is a '51 with a 261 (1962 version) I used Patrick's short shaft pump and his fan with no clearance problem. With the spacer on the fan pulley the blades cleared the crank pulley/harmonic balancer. I threaded the crank to use a small block chevy crank bolt to press the pulley back on and the fan didn't clear the bolt head so I got rid of it- I don't think it needed to stay in place once the pulley was in place. Patrick's instructions say to 'tweak' the fan blades so that it'll clear the crank pulley.

Have you test fitted the fan/pulley and pump?

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi