Need advice on rust issues on a 68 Panel

[68PanelTruck]

Hi guys,
I picked up a 68 Panel truck and need some advice. The truck has a lot of rust, some that I can fix and some I'm not sure what to do. Things like door skins and rusty fenders I'm fixing now. The cab floor will probably be replaced with a new pan (full pan with braces). It needs some frame/body mounts replaced.

The areas I need help with what to do is the barn door sill area. It is still there and functional but is almost rusted through. I'm not sure if I could even weld a patch to it as it is very thin from rust. I wish a new piece was available. Should I look for someone parting out a Panel Truck sill and try to replace it? I'm not sure what my options are.

Another area I need help with is the where the cab floor meets the bed floor area. The cab floor pan sits on or is spot welded a crossmember(?) spanning the width of the truck. This crossmember is also rusty and may have to be replaced. Not sure until I remove the cab floor. Again, I wish I had a good replacement but....
Should I be looking for a donor crossmember or try to fab it myself?

So here's the deal. This is a Loooong term project so I'm not in a hurry to knock it out. I have a lot of experience with the mechanical side of things but bodywork and welding are new to me. I'm a "figure it out as I go " type of guy and I'm doing ok with welding in the metal patches I've done so far so I believe I can do this myself. I'm not scared to try to fix this, just need a little direction. I know it would have been easier to find a truck in better condition from down south or out west but that's not the point. This is what I have and I'm up for the challenge ! I really bought this truck to learn new skills and practice on it before I get a nicer truck down the line.