Seats/saddles are a highly personal choice. Nobody has the same sitbone position or comfort level. Most saddles can be used with the old style clamps, so there shouldnt be a problem reusing the old clamp on a modern rail seat.
Many recommend Brooks, but I have at least one or two Brooks saddles (both B67) that I had to put on an Avenir med memory foam cover on so I can ride in comfort for an entire day, not just an hour or two. More than a few like the wider "cruiser" saddles for max comfort and a sprung saddle is preferred for upright riding regardless of saddle sizing.
May have to try a few saddles before finally finding the right one for you. Have an inexpensive Schwinn pillow top saddle on my beater bike and I can ride on that all day. Only lasts a year or so but I ride 365, so annual replacement is to be expected for a saddle in that price range ($20).