I replaced my freewheel on my 1980's Basso with Campy components, for similar reason, needed lower gear range to climb hills.

One thing you may want to check into is if a 28 gear freewheel will fit with your derailleur. I found that a 26 gear ring was the largest cog that would fit on my Campagnolo rear derailleur.

I switched to a DuraAce 6 speed freewheel 13-26, which works really well.