[Big Dave]

The GM 6.5 is almost a bolt in but gives a more crowded engine compartment. The GM 6.5 and the Cummins 3.9 are about the same weight - 700 pounds.

The Cummins is taller and may require some hood mods, but I'd just do the same hood reslope that Phil (Aerohead) did on his T-100. The Cummins would require the motor mounts to be relocated.

Either engine will require body sound deadening material.

Not only does the Caprice have a better Cd, but also a slightly smaller frontal area. The pickup has a roomier driver's position and a more upright seating position.

Much to consider.