Read through it and have similar situation. My answer that seems to fit
the bill is to use spray foam (they recommended rigid). Tigerfoam.com
sells it for DIY.

dpb wrote:
buffalobill wrote:
lots of reasearch already done for you at:
http://www.buildingscience.com/resou...ns/default.htm
...

Which near as I can tell, gives no advised solutions for insulating
basement walls on the interior side, not very helpful for existing
construction.

Worth reading, though, as in humid areas particularly, vapor barriers
on basement walls can do very bad things. Not thinking about having
not had a previous basement that was finished did the vapor barrier on
the wall in TN and on the fully underground wall it did end up w/ very
bad moisture problems and failures in a very short time...

I don't really know what would be best solution overall, but agree w/
someone else that the air space behind is probably a better idea than
mashing the 6" into the cavity.