i've researched this for a sua design. they are wider springs than the 40's, with bigger bushings, so you either can use 60 series spring mounts, or widen your 40 series mounts. they are longer, with a different pin offset, so you will gain wheelbase, and they do flex well, although in the lighter 40, you may want to look at taking a leaf or two out, depending on your setup, but knock it together as is, then you can play with getting the right amount of leaves.
apart from the difference in width, pin size and bushings, it's the same tech involved in a 60 series spring swap as a 55 series.
only use the rear 60 springs, as the front springs are about the same length as a 40's spring, gaining you nothing. but you could use 60 rear springs in the front and rear of the 40, as woody has done with the 55 springs