This page,
http://amiga.resource.cx/exp/search.pl?product=AdRam540&company= , says the interposer goes under the Gary chip (Uh-oh).
Seriously, I think I would gently pry up all the chips most of the way, sparingly apply contact cleaner, and reseat. It might not hurt to check for swollen caps and make certain the battery has not leaked and contaminated anything.
If you have to replace ram chips on the MB, maybe consider upgrading to 1 Meg of chip ram. The AdRAM seems to allow this.
