A photo of your set-up would be a great help, but two more points....

The belt must go around the water pump at least 5/8ths of the way, or it will not have enough surface contact. Your description suggest only 1/2 of the pully is in contact, and your belt is on the wrong surface.

The belt only has groves on one side. When it goes around the water pump, is the groove side in contact with the pump, or the flat side?

If you had a power steering pump, I think your set-up would work. But you don't. I think mininum you need 4 pullies and one idler.

Sounds like you may need to cut your losses, and get a regular V-Groove set-up.

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66 Stang 385+ HP 306 .494 /.520 225 durr @.05, 200cc Windsor Sr's, Edle RPM, C4, 3.00 posi, 575 Annular Mighty Demon, 22 MPG,