Thanks, Jeffry. Yeah, that's a logical location and, as long as you don't get too long, it keeps stores clear of the prop-rotors. I wonder, though, why they didn't use a stub wing instead of just an "erector set" rack? (*wink*).
I'm told that the MH-53K engines have been looked at, but considerable redesign would be required to fit them in the Osprey (accessory and drive section) and it's just not cost-effective at the moment.
An extension of the LH sponson for additional tankage is possible while extension of the RH sponson is limited by the presence of the crew door. There's another approach to conformal tankage under study, but more I can't say. It's the obvious approach, though.
To fit the CH-53E tanks, you'd need some new structure on the top of the sponsons, including a fairing to extend out and keep the tanks clear of the main gear.