Without assumptions or advise, you may want to consider Microshifters. Less money, and appear to have acceptable reputation, so far.

There is a possibility that you may not have to do much to move to ta 7 speed. Check the clearance between the chain on the smallest sprocket and the seat stay. What is more important, is having enough room for the chain to climb the teeth when shifting. Although not a 70's bike, my upgraded 2x6 to 2x7 only required a freewheel change.

6 sprocket block:
[IMG]P9161258 on Flickr[/IMG]

7 Spd Block:
[IMG]PA121374 on Flickr[/IMG]

No adjustment made except the RD limit screws.