My old Trek touring bikes that still have freewheels all have seven speed freewheels, which should fit just fine on your wheels, and are set up with a half-step plus granny. This gives one a great range plus tightly spaced gears at the cost of doing a lot of front shifting. A typical touring set up would involve chain rings of 45-42-24 and something in the 12-24 range on the back. One of my old Treks has been modernized by cold setting the rear dropouts (after the seemingly inevitable failure of the seat stays at the seat tube; be sure to check for that regularly on your 620) to accept a nine speed cassette. That has become my favorite bike.

If you have a full-service hardware store nearby, you should be able to get rubber-coated U-bolts to attach low-rider panniers on the front. A bike really does handle a lot better when you put the bulk of the load even with the front axle.

On brakes, you can replace yours with some more powerful dual pivot side pulls, which may restrict your tire size a bit, or bite the bullet and find a framebuilder who will put on the appropriate braze-ons for the modern brakes of your choice. Of course, if you choose to go disc in the rear you're going to be building some new wheels and cold-setting that rear triangle. I'd probably put this sort of thing off until the seat stay/seat tube junction fails. By that time you will know if you love the bike and the way it fits you or not.

Edit: Are you sure there isn't a second water bottle braze on under the down tube just forward of the bottom bracket?