Can a 6 lug rear end from 1955 2nd series GMC truck be converted to 5 x 4.75 bolt pattern?

No, you can't directly convert a 1955 GMC truck's 6-lug rear end to a 5x4.75 bolt pattern without significant and likely impractical modifications. The fundamental problem is that you'd need to:

* Replace the axle flanges: The 6-lug axle flanges are integral to the axle housing. You can't simply change the bolt holes; you need entirely new flanges. Finding replacement flanges with the 5x4.75 pattern that match the 1955 GMC axle's dimensions is extremely unlikely. Even if you find suitable flanges, precisely matching the axle shaft diameter and spline count will be a challenge.

* Machine work: Even with new flanges, it's probable that the new flanges won't directly bolt onto the existing axle shafts. This would require extensive and precise machining.

The cost and difficulty of these modifications would far exceed the cost of sourcing a suitable 5x4.75 rear end from another vehicle. It's simply not a feasible project for most people. It would be much easier and cheaper to find a suitable replacement rear axle assembly.