Where is there information indicating that a twin turbo 7.3 or 6.8 in development?They are working on a twin turbo Godzilla or a variant of a 7.3L, possibly a 6.8L. To answer your question they already have the Predator engine available, adding Nagasaki Noisy Boyz to a higher displacement V8 and achieving long term reliability would take considerable testing but it would decrease fuel consumption. Ford wants to go out with a bang on V8 supercharged engines to compete with the outgoing TRX or variant.
Here's what I think: I would dare say the fuel economy would be around 8-10mpg in town and maybe 14 mpg on the highway for the Predator in a Raptor body. The only time you may see 400 miles to E with a 36 gallon tank is leaving the dealership.
The advantage the Raptor R will have over the TRX or future Ram TRX Redeye is both weight and the ability to have RWD to 4WD or 4A vs the TRX is essentially AWD and that's a lot of mass to moving. I would also speculate the Raptor R would be easier to work on as it probably won't be as deep in the engine bay as the Hellcat motor.
The Predator in the GT500 takes 11.6 gallons of oil, I think a truck variant would have over 12 quarts or more especially if any of the coolers are increased in size. I would prefer a console shifter to a dial.