I needed a two-sheave pulley for my water pump.  I searched and could not find one anywhere. 

One day I was messing around in the shop and on a whim decided to pull the pulley from the V8 sitting in the corner.  When I laid it on the bench next to the single-sheave pulley from the 250 there was only a small (about 1/4 inch) difference.  If I made a spacer to go between the V8 pulley and the water pump flange the V8 pulley would work.

I made a template from cardboard that had the hole centers marked as well as the diameter of the spacer. 

Happily I had some 1/4 inch aluminum plate in my metal pile and marked the holes and circumference with a center punch and paint pen, respectively.

I drilled the center to fit the boss on the water pump, then the bolt holes to attach the the pulley to the water pump flange.  The last step was cutting the spacer to shape.  My woodworking bandsaw worked well for that and my drum sander helped me get the thing mostly circular.