There is a voltage regulator adjustment procedure in the shop manual. It's kind of a fiddly process, done by carefully bending bits of the relays. It's a shame that a new one needs adjustment right out of the box.
Hotrod Lincoln will tell you that if it isn't an original Delco-Remy regulator, it's junk. But maybe you can make it work. If you don't have a shop manual, you can check
HERE. It's the 1954 shop manual but should be the same as your 55.1 truck.
