NAD C 328 on-end placement

You should be fine,  Note the best idea but no going to damage the unit other then maybe scratch the side up a bit. 

Make sure you still have plenty of space around it though. At least 2".

The NAD hybrid Class D amps run hot compared to other Class D amps due to a linear amp in the middle. Relative to linear A/B amps, it’s cool, relative to other Class D amps they run warm.