I have two bikes - one a vintage 12 speed with down tube shifters - the other a twist shift gear conversion with 21 set shimano gears. I want to put the 21 gear set on the intage road bike - can I simply connect the downtube shifters in place of of twist shift?
You're going to have a few problems most likely. For example, you need to probably respace the rear dropouts to get a 7 speed wheel in (assuming its a right size/close enough size that you can get the brakes to work). Then you have to make sure the triple clears everything on the new bike. Then, you have to make sure that the FD can be mounted on the new bike and can shift a triple (they're different between doubles and triples) and the RD can move enough to get through all the gears (which also probably means swapping the RD's).
Once you have all that, with friction shifters, all you need to check is that they can pull enougyh cable to shift the FD appropriately and the RD appropriatley (i.e. from small to large).