Thank you all very much! Based on your advice I'll either track down a 39 tooth front chain ring or a 6 speed freewheel with a 28 tooth large cog to start with.

If I did want to "upgrade" to the 7-speed setups of that era at some point, would my current 105 derailleur be able to handle it, or would I need a new rear derailleur, shifters, and freewheel?

If I'm remembering correctly, I think the current rear freewheel is a Uniglide. Should I be looking for a 28T hyperglide freewheel to improve shifting performance? Were these even made, or are they all cassettes?