restoring 73 Impala...?s

I picked up a 73 Impala sports sedan recently..dirt cheap,I'm in the middle of a frame off resto. The problem I see is rust(some serious some minor) in some kind of hard to reach areas...the majority of the rusted steel I'm cutting out and replacing .My questions are what about tight areas like between the roof skin and roof inner shell.What about the backsides of this new steel...how can I prep/protect the car from rusting again???