Your best bet is probably to take the old hose down to your local parts store and compare what they have in stock. They may even be able to identify it from your truck description.

Rockauto has the following for a '66 Medium Duty Truck

DORMAN H36697
Fits Front Left; C60; 25 In. Length; 7,000 Lb. Front Axle
or Front Right; C60; 25 In. Length; 7,000 Lb. Front Axle


Kevin
1951 Chevy 3100 work truck
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Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.