Sometimes I think English is my second language...
The issue is not the selector itself (I used the Van's supplied one rather than the Andair) but the structure around the selector. In the -6 that consists of a couple of vertical pieces (sorry, I don't have the plans at the house so I can't give the callouts) that support the fuel selector valve plate. They angle inboard somewhat and are bolted to the spar. On the -7, there are the F-783B-R\L that get riveted to the center section, are vertical, and ultimately mate with F-983C Fuel Valve Cover which, in turn, supports the fuel selector valve plate. They can be easily modified to fit in the -6 and make a nice, enclosed fuel valve installation, as opposed to the open one created by using the -6 parts. Better still, you can use RV-7 Drawing 34 to get the other parts forward to the firewall to enclose the fuel line out to the firewall; these should fit the -6 presuming you have used Van's firewall recess and not a steel bowl recess. This also includes an enclosure and mount for a boost pump on the firewall (if you are not using injection). None of this is a pain to fit (sorry I left this impression) if you do it while the fuselage is still a canoe. Otherwise, you will be diving under the panel. The advantage for non-fuel injected installations is the enclosed run from selector to firewall.
The real advantage is for the fuel injected installations using Van's FWF kit. The pump/filter cover was designed to mate with the -7 parts (vertical), not the -6 parts (tilted inboard). If you've used the -7 parts to begin with, the pump and cover installation is fairly simple. Otherwise you will need to change to the -7 selector mounting parts or kludge a cover to fit the -6 selector mount. I did the latter and it was not pretty; I may yet change my installation.
By the way, I mention RV-7 Drawing 34 but the F-783Bs are on Drawing 23 only, which you don't really need because the structure is so different from the -6. On 34 they are shown beneath the F-782A cover plates, which you should omit on the -6. I hope this wordier description hasn't caused more confusion. The best advice I can give to -6 builders considering this is to get a copy of Drawing 34 and, if you are going fuel injected, the drawing for the pump installation kit from Van's and compare them to the RV-6 plans. It should be fairly easy to see how to make it work.