Rough situation. On your bike, you need to get a Campagnolo rear cassette in 8 speed, which they no longer produce. Are you sure what the difference is between a freewheel and a cassette? I'm not a vintage campy expert, but I don't think they were producing 8 speed freewheels in 1994 (or ever?) Unlike with Shimano, they don't produce low end parts, so they don't really make 8 speed drivetrain parts any more. Also, if you buy new old stock products from that era, they're going to have a relatively tight spread of gears. Also, your rear derailleur isn't probably designed for bigger than a 26t cog--you may be able to get it to adjust to a very slightly larger cog, maybe 28t.
Miche Primato M Drive 8 speed cassette Campagnolo compatible | eBay would be your best bet, but you will likely have difficulty setting up your rear derailleur to clear the largest cog.
Switching to a compact crankset may be more fruitful/easier.