I'm considering buying a '58 F100 with the straight 6. Problem is that the auto trans needs to be replaced. Are there compatible transmissons from other models that would fit my F100 (straight swap)?

 

If that's the original engine and transmission, it should be a Fordomatic. And according to my '59 shop manual (and common sense), there's different versions for the V8 and sixes. So, I'd guess your options are a transmission from a car up to 1959, and trucks up to 1964.

It could be safer to rebuild the transmission you've got, since a Fordomatic out of a wrecking yard or scrapped vehicle could be worn out or busted. Or convert to a 3 or 4 speed manual. However, trucks with Fordomatics are rare, so you may want to keep it original.