Did you have an engine before you bought the crate motor? I just moved all my stock brackets over to the new crate motor.
My '75 350 had brackets like the picture in an earlier post, except I have an A6 compressor and I think the one in the picture is a R4 compressor. My original compressor and brackets were missing, so I found another set of brackets in the junkyard. GM used the same bracket and belts setup in cars, so there were a lot of vehicles to pick from.
You also need to make sure you get everything with the right number of grooves in the pulleys. I think the crankshaft pulley has three grooves (alternator belt, PS pump belt and AC belt, the water pump has two grooves (alternator belt and PS pump belt, and the compressor has one groove.
With AC you should use a longer PS belt that also goes around the water pump, but without AC the PS belt only goes around the crankshaft. This means that when you have AC the water pump is being driven with two belts, which keeps the cooling system working if either the PS belt or alternator belt breaks.
Bruce