If brifters are not an absolute, you can get a rear wheel with 8,9 speed hub and the DT shifters, in friction mode, will work through the gears no problem. If the levers are 6 speed index only, it will not work. You can get a doublewall wheel with 8/9hub for under $75, a single wall for $50 or so.
91 Ochsner using 9 speed wheel/cassette with DT shift levers.
I did this with a rebuilt early 90s steel bike. I also cold set the rear spacing to 130 mm so the wheel would fit in the drop outs properly. I have yet to reset the drop outs but will do soon. Both not hard to do. Video by rjthebikeguy.