Which conversion you go with really depends upon the price of each fuel in your area, its availability, and how much fuel you go through annually for its payback.

There are V10-specific LPG and CNG kits available but none are EPA-certified so they would be difficult to be commercially installed in the USA. CNG systems are generally not available to the DIY installer so CNG conversions are even more difficult. I have no idea if there is an EPA-certified E85 kit available but I would expect that it would easier to DIY install it.