I had a 312 (same bike with index shifting from '86 or so) that was run over. I still have the bars and seatpost. PM if interested.
As stated, 6 speed freewheels can be sourced from several places. The Nashbar ones are made by Sun Race, and I've had good luck with a Sun Race 7 speed on another bike.
If your comfortable with friction shifting keep the 105 gear-they don't come much better in terms of function. BIIWM, I'd look around for a cheap old index RD (like a Light Action) and shifters and convert this bike to index 7.
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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.)