Sounds like the brake system is contaminated with oil. If the wheel cylinder rubbers have grown to twice their size do as an earlier responder mentioned flushing the entire system. I would just replace all the wheel cylinders anyway.. be safe
Not 4 wheel drive but I put an nv3500 into a 1980. Used the flywheel that was on the 93 305 it was mounted to originally. I also used a new master/slave hydraulic system from that same year. That might just bolt up your you existing clutch bracket..
They recommend notching the bed support bracket. I put these on a step side and didn’t notch the support. It does not hit but is very close...will probably do it before I drive it a lot.