It should work on a Mac Plus with an old enough configuration of the disk operating system, but at some point with later models or upgraded operating systems the support for 400KB single sided diskettes was ended. If you take a blank new 800K diskette to begin with, and initialise that as a 400K, somewhere in the disk initialisation process the dialog box will give you a choice of formats: 400K versus 800K. If you do not reach this dialog box there is something changed from the original Mac Plus, and you need to solve that problem before attempting copying the 400K original software (I think).

I suggest you try to find and buy a box of brand new double density (720K or 800K) 3.5" diskettes of high quality, if they are still being made.

I suggested you use a system with two diskette drives to avoid having to reload each of the source and destination diskettes several times into the one drive during the copy process, because a 128K Mac cannot copy a (full) disk of 400K size in one pass. I do not know how much RAM a Mac Plus needs before it can copy a 400K diskette with only one pass.

You are also better off doing this in a system with two external diskette drives or some other combination that avoids a situation where a defective source disk gums up your systems boot 3.5" diskette drive (if any).