Forget cutting a piece from a donor car, thats like doing the job twice.. In this case, shape is not critical, as long as the holes are in the correct position, and the hinge surface area is at the correct angle, duplicate whats already there, by cutting a plate slightly larger than the rusted area, form the "squared edges" with a vice and/or molegrips, lay your new patch over the top of the rusted area, hold in position, and scribe the mounting holes from underneath, remove, and drill holes in the new plate, now bolt your new plate into position, scribe the outer edge, remove plate, and carefully cut out rusted area, up to the scribe line, remove paint ready for welding, refit the plate, and bolt into place.. tack weld plate into position, and recheck that the tailgate still fits... fully weld into place, dress with a grinder, fill as required, go to pub etc etc
Tom
