"For the time being" vs. "for now"

Consider the following passages:

A litter made of two rifles and two field jackets would suffice for now. That was good news; another bit was that the EPW was a lieutenant, a regimental REMF attached to a battalion quartered in town. --Rangers Lead the Way By Thomas H. Taylor, Robert J. Martin

He had a couple of thousand dollars on him that would suffice for the time being, until he could find some kind of legal employment. He got out ofthe car, and walked a block. --The Maze By Kahn Morris

Now, how are these expressions different? Could we use them interchangeably? Oxford Dictionary of English has the following definitions for them,

for now until a later time: that's all the news there is for now;

for the time being for the present; until some other arrangement is made.

If you are operating in the present tense, you can definitely use them interchangeably. I can find no definition that implies further semantics to distinguish between the terms.

However, outside of the present tense, "for now" might sound weird. It may be better to refer to "the time being"

Tomorrow we will pick up one bag of groceries, and then that will have to suffice for the time being.

However, "for now" could definitely be used in another tense if it reflects a subject's point of view; even though it happened in the past to the reader.

A litter made of two rifles and two field jackets would suffice for now. That was good news; another bit was that the EPW was a lieutenant, a regimental REMF attached to a battalion quartered in town. --

'For the time being' is a more fluid expression, while 'for now' is more static. Choice of one or the other would depend on something other than strict meaning. Mathematical tables would be unlikely to uncover the subtleties of the choice.