[dsraven]

The part you likely need is a door hinge pillar. There are a couple parts down there and they all seem to be sandwiched and spot welded. The front cab mount is also part of that.
Water builds inside the inner cowl and runs out on the cab for. The drain in the cowl bottom is small so it plugs easy. The water on the floor runs into the void in the hinge pillar and rots it all out. Then the front heavy cab sags down and usually makes the floor uneven and low under the front pedal area on both sides. The step and rear door pillar bottom are the next victims.