I never heard of a 4.89 in an early dodge ?? that would be compatable with your axles. My 3.9 spare out of a 52 Desoto has the course spline. If you are talking about the 4.89 Dana in the later model metals, I don't think the axles would work. I think it is a totally different set up. Going from 4.1 to 3.9 for me will be ok and may even be better for gas mileage. Won't make much difference either way I suppose. At my age a little howling doesn't matter much, getting deaf so I may never have to use my spare but if I don't keep it in the shed , sure as hell I'll need it.