"86 Cannondale" That says friction shifting to me. It means you don't have to worry about fancy chainrings or which rear derailleur will work.
If it was my bike, and I was looking for a serious hill climb gear, I'd go for the "Mega range" freewheel and a different rear derailleur. Since you want to keep it (sort of) original, I'd look for a Suntour Cyclone long arm rear derailleur if I could find one or even a classic VGT. Otherwise a basic Shimano mountain bike derailleur will function just fine. Messing around with a 28 biggest cog vs. a 24 biggest cog is only a baby step. It might give you the range that you're looking for but I doubt it.
Being an un-racer, I've developed my own philosophy about gearing. When I'm on a basically flat surface I like to have lots of gears spaced closely together so that I can make minute adjustments for wind conditions or small inclines. Once I get into the hills I want big gaps so that I can feel the difference when I shift. I don't want to have to shift too often either because every shift causes me to lose momentum. There must be a reason why the mega range freewheels and cassettes have been as popular as they are for such a long time. It might be because there are lots of folks like me.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.