On my bike I'm running a similar setup, it's all Shimano 600 tricolor minus the freewheel which is Shimano Drua-ace with 7spd indexed DT shifters. If you can find something nicely used you would probably be better off than anything new. Search MF-7400 on eBay, there is one on there right now.
EDIT: Setup shifts really well, haven't had any problems with it at all.