Tubes?

[Whoops]

I ordered new Angels for my EX, and understandably, pretty much everyone requires new tubes be installed when mounting new tires. So I ordered the Honda OEM tubes front (110/80-18) and rear (140/70-18). 

However, the rear tire I ordered is a non-standard tire size at 160/60-18. Will the Honda OEM rear tube that is designed for a 140/80-18 work with a 160/60-18 tire? The parts guy at the dealership where I ordered the tubes said it will work just fine and will fill the space. However, I get the feeling he wasn’t as knowledgeable as he purported himself to be after he asked why I am going to put “the wrong tire size on the rear”, even after explaining how hard it is to find tires for this bike.

Should I be trying to find a tube with the same dimensions as the tire I am mounting?