I recently bought a 1986 Cannondale SR500.
It has the original Shimano 600 MF6208 13-18 freewheel.
I would like to use an MFTZ20 14-28 freewheel (full part # is EMFTZ206428T).
Will that work? If not what should I use to get a larger freewheel?
Should I change the chain too? What chain?
Thank you for your input.
Jim

I assume both freewheels have the same number of cogs (6 right?) If that's so then the 14x28 will be a direct replacement for your 13x28. You should change the chain because the worn original chain is very likely to "skip" on the new cogs. Any Shimano or SRAM "8-speed" chain will work fine and the SRAM comes with a master link that makes joining the chain easy.

Whoa, hold on - OP said original freewheel is 13-18 (not 28) which fits the model #. To move to a 14-28 you absolutely need a new chain, sized to fit properly. The Shimano 600 derailleur will handle 28 teeth and will handle up to a 14 tooth difference between the front chainwheels. Go to Sheldon's site for sizing and installing a chain.

Whoops, yes I missed that rather badly. Sorry. And also yes, a new chain will certainly be needed to cover the extra wrap and because a new chain with a new freewheel/cassette is almost always a good idea.