I've toyed around with several tandems in the hope of making one work as a solo, but without much luck.  I have a Bell MorningStar (15'6" x 35") that I made narrower, but I haven't tried it out yet.  A better choice might be a NorthStar (16'6" x 34").  It's a little narrower to begin with and could probably be made another 2" narrower without overstressing the hull.  I know of an experienced BW tripper who uses his as a solo and says it works just fine.

As for paddling position, I feel for you having to give up kneeling.  If you can kneel with one leg, maybe you can stretch the other one forward.  If not, I think you can still lean the boat, which is easier if you're comfortable leaning your leg against the gunwale.  I paddle mostly solo, both small local rivers and BWCA, with a mix of kneeling and sitting, and whichever position I use I lean the boat to the onside.  It's just a little more secure when kneeling.  You should be fine.