My understanding is that standard J frame grips do not fit the 40/42 models...that is what I have read on this forum.
I must say I like your thinking...if you have a spare set of grips, don't sell the grips...buy a gun to put them on! My wife doesn't see the logic in that, but it makes perfect sense to me!
The old Model 40's / 42's have a 1/2" diameter washer on the inside that are unique to this frame. The backstrap is slightly different contour also.
Standard J frame grips will not fit on these guns.