The main pivot plate can be cut and drilled manually, but for accuracy and cost effectiveness, it is better to get your local profile/laser cutter to fabricate them for you.
Tack weld the main pivot plates into position on the pivot beam making sure they are square to the beam and the holes are aligned correctly (all four holes aligned and parallel). Making and fitting a 55mm packer to fit between the plates while welding, as well as a 1900mm long, 16mm solid bar, through the holes, will assist with alignment.
Take your time setting this up as a poorly aligned pivot beam can cause uneven pivoting of the trailer deck and possible jamming of the pivot arm.
Fully weld the pivot plates to the beam. Stagger the welding to prevent heat distortion and twisting of the plates and beam.