I'm not sure the aft tank would be suitable without modification (I'm not on the boat right now, so I'm working from memory...). I believe the top of the tank onboard JUNO (PSC 40 -- #46) is either plexiglas or Lexan, which I don't believe would be usable with diesel. So, at the very least, you'd have to install a new tank top.
I believe the bottom and sides of the tank are fiberglass, which, I think, would work, but someone with greater knowledge could probably advise you.
On JUNO (PSC 40 -- #46), the number 2 water tank, located beneath the sole aft of the galley, is stainless steel. Stainless can be used for diesel tanks, but I've read it has pros and cons (Some say it's great; others say the welds are subject to corrosion and will ultimately fail). But that tank might also be a possibility.
On 40s that were equipped with a dedicated second diesel tank, it was located beneath the dinette seat just forward of the galley, which put it quite close to the primary fuel tank, and there was a manifold installed there to control the feed of the two tanks. If you are willing to give up some storage, this, too, might be a possibility.
Perhaps one of the 40 owners who has a factory-installed second tank will weigh in.
Hope this helps a bit.
PSC 40 -- #46
(Still on the hard in Rock Hall, MD, but hoping to finally splash the boat tomorrow and start commissioning...getting a very late start this year)