I want to convert my 76 f250 to a dually which would be better to put a dually axle or buy adapters ? Which fenders should I put ? Any help

 

Dually axle is better IMO... purpose-built and engineered.

Fenders? ... consider the fenders from a 55-66 Chevy C-10 stepside.

 

What a cool idea!

 

What kind of modification is there if there is any? what kind of axle? I like that idea of those fenders .

 

Depends what axle is sourced... A dually Dent axle would be ideal. Grab the leaf springs. Also the driveshaft if it matches your driveline configuration.

Otherwise, determine potential donors by researching axle widths, leaf pad to leaf pad dimensions, leaf spring widths, pinion yoke/u-joint sizes, desired gearing, etc.