In my mind, the biggest thing these two vehicles have in common is the current 5.0 V8; although the Mustang GT version is tuned more towards power and performance, whereas the F150 is tuned for a flatter torque curve and for more durability for working harder more often. Rumor has it, and at least one official has confirmed that there will be an all-new V8 4.8 direct injection to replace the 5.0 for both the truck and GT. I'd suspect that these two vehicles house test versions of that new engine and that plus the new ten speed, most likely also in both vehicles, is why they are seen together. It's likely power train folks running test trials together in these two vehicles, and it's likely that Ford Motor Co. desires to be caught by spies and is why they keep getting caught! The mid-cycle refresh and the diesel probably have little to do with this particular spy shot.
On the subject of the diesel, is Ford going to keep all four gas engines plus the diesel, or let one gas engine go? The only one it makes sense to drop is the base V6 at only a 10% take rate, but doing that puts them at a cost disadvantage on the base work truck versus Chevy, GMC, and Ram who all have base NA V6s. On the other hand, offering five engines versus their three puts them at a cost disadvantage as well one would think. There is no way though that either EB goes away. The 2.7 was new for '15, and the 3.5 is new for '17. They're working on a new V8, and the current 5.0's got almost a 30% take rate, so all three of them have to stay. I personally think it's cool if F150 has no base, weasely, NA V6 and makes an EB the base engine, but competing on price with a more expensive twin turbo and an all-aluminum body might be tough w/o losing margin against the competition.