I really like those 6 speed 105 shifters. I'm doing a similar upgrade to yours by building new wheels (since I've never done it before thought I'd do both and do the supposedly easier front as practice - also have it complicated by needing to have 26" wheels with 130 mm hubs), but I'm planning on staying with my 105 shifters in friction mode for now. I decided to go with Kelly Take Off's:

as a way of mounting my shifters to make them more accessible rather than on the downtube. So far it's a setup that's worked really well for me. Others might correct me on this, but it might be better to go with am 8 speed cassette if you're staying with your current crankset from a 6 speed (though that does get back to potentially limiting your cassette choices a bit again). If you did decide to go with 8 speeds vs 9, don't get the currently made shimano 8 speed indexed dt shifters as they have no friction option and I'm a big fan of the friction option when something or other has bumped your RD out of whack (in my case being left on the rack while I'm at work and somebody next to you isn't careful with your bike) and you just want to get home before trying to adjust it. I suppose even if you did get the 9 speed dura ace dt shifters (which do allow friction), you could use the indexed if you use the alternate routing of the cable and set your limit screws carefully.