I replaced the freewheel on my '87 Ironman Expert (105 rather than 600) with a seven-speed SunRace. I've run their freewheels on several bikes with good results. Yeah, they're 'cost effective', but they work.
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You know it's going to be a good day when the stem and seatpost come right out.
(looking for a picture and not seeing it? Thank the Photobucket fiasco.PM me and I'll link it up.)