OK. So it is a physical spacing issue. I wasn't sure because I dissassembled one of my old unworking cameras before (Minolta) and the IR filter was pretty easy to remove and wasn't mounted such that it acted as a spacer.No, It isn't only a matter of distance and spacing.
Because of the refractive index (n) and the thickness of the glass, it's an optic issue, there is a formula to find the difference in focus:
OutOfFocusDistance = (n - 1) / n * ThicknessDifference
(you can find explanaition in many page about cameras mod like this: http://astro.ai-software.com/articles/mod_350D/mod_350D.html
Furthermore you have to consider the back-shift of the focus as the wave lenght increase, so even with the same thickness in the replacement glass you will notice different focus distance (infinite focus come earlier); in fact in some cases a replacement glass just a bit thicker may balance the opposite trends and give a near perfect focus...