if the 51 is the same as 55-59, the slot in the pillar the hinge goes in is 'sloppy', and the bolt holes in the sheet metal are oversize, so there's adjustment possible [certainly 1/4"], ditto where the hinges bolt into the doors .... but it could also be the case that the doors you have just don't fit the opening well, a common problem with using parts not original to a particular truck, at the factory they could chuck that set aside and try a different pair, you're stuck with fudging it as necessary, shims might be part of the cure .... check your shop manual
Cab and Body section, barn door adjustments should be near the same as side doors
Bill
