I would cut a small section from the outside so you can better evaluate what you are dealing with.
You are probably right about dirt plugging the weep holes and retaining water. Fortunately they still used fairly sturdy steel back then.
I can help you with that rust issue, sell it to me for a profit.
Careful what you say... .... Doing a once over and there are some bad spots in the cab too... someone sprayed this foam crap all over the inside and water has gotten in behind it along the edges from condensation and made a mess of the metal... It's not hard to fix them it is just there are way more spot welds to drill than I was wanted to deal with again. Easy a week or two just doing that.... May be easier to just sell it for a profit!