T-Mar hangs out more in the Classic and Vintage forum but he might chime in here.

I'm not sure what you mean by loose cranks, though. Perhaps the bearings in the bottom bracket are worn and that's what's making things feel loose?

And switching from a 42 to 39 tooth chainring won't have much of an impact on climbing. You could get a much bigger change by swapping out the cassette or freewheel on the back for one with a larger sprocket.