I have seen some older chassis in the lab where there was a very tight fit. I have also replaced a chassis for a customer and that resolved the fitment issue. Did you have an older NPE model in this same chassis?
The NPE-G2 still installs into the NPE slot on the chassis (the side with the power supplies). I have also seen a Service Request where the problem was the customer was trying to install the NPE-G2 into the I/O slot.