The closer you are the worse the convergence will be, so as MMM said, back up with a 35mm or longer, if possible.
Software stretches pixels to obtain a shifted look, so that should be your last, (or cheapest), option.
A tilt/shift lens is the obvious best solution, but they are not cheap - about $1,000 used for a good Canon. Samyang is a bit cheaper, but you know what they say (I hope)...
And don't forget the depth of field you can play with using the tilt function - hard to replicate in software.
Here is a sample of 17mm on full-frame - much wider than the common 24/28mm on a K-3. Note I do not totally correct the vertical as it will look unnatural - the eye expects some convergence, and if it is totally straight, you will perceive it as bulging outward....
http://specialk.slickpic.com/albums/UCLA/?grid#10140811