eBay is full of 1980's low mileage 13-28 and 14-28 UG 6 speed freewheels that last forever, will fit on your bike, and interface with your indexing system.
Prefer new or like new? Check PastorbobinNH or his website the Freewheel Spa. He'll make one for you at a reasonable price.
You will likely need a new chain or at least a few additional links if yours is new, but everything else will work together just fine.
Note to self: ALWAYS keep any extra links from a new chain that is shortened to fit.
6500 double pivot brake are an upgrade, if you can get them cheap. do it. Everything else won your bike will work with them.