Why wouldn't a 7 speed freewheel work? I'm not sure how much wider it is than the 6, but if the der has the range of movement, you should be able to adjust it to work with your current friction shifters.

Also, if your rear span is 126, it should go right on. I'm not an expert but I can't think of any reason why it wouldn't.

7 speed freewheels can be had new at Nashbar for less than $20. $10 on sale. Might be worth trying.

55/Rad