I am going to build a 5.7 for GT and can offer the following. I have been told by NHRA that if an engine has fresh air (scoop) in its normal class then it has to be used in a fresh air car in GT. The opposite is also true, a non fresh air car can't be used in a fresh air car. So the Drag Pak engines in GT has to be in a scooped car such as a 68 hemi Cuda or Dart. Since the rule book says that the GT truck class has the same rules as the GT car rules with the exceptions listed in the GT truck section it would look like the Drag Pak is not legal in a truck. Don't know of any truck with fresh air. I'm using a 68 AH type Cuda.
As for the 5.7 and 6.1 engine, the 2009 engines both came with the 6.1 SRT8 heads with slightly larger valves, casting#5037369AA. 2.10 and 1.60 valves. The pistons are 12.7-1 Diamond #46191. Has a hyd roller with .584 and .552 lift. Non adjustable rockers.
I haven't started my engine yet. Have had some shocks on parts. Jesel drop in roller lifters for $3200 and rockers for $4800. Harland Sharp does have a set of rockers for $1800 that are adjustable. Think the rods are Scat with ARP bolts. Manley and K1 make approved rods.
As for the HP factors, the SS and GT factors are seperate so it would take a GT car to hit the GT factor, not a SS car.
Good luck with whatever direction you go.