almost did the same thing you did but they want to put the 3.8 into the truck.
just drop it right in. if it's a 5spd then you need the 3.8 flywheel to make sure it's balanced. Not sure on the neutral balanced 4.2 years.
you will need to swap the oil pan from the 4.2 as well as the TB and upper intake but everything else should be OK. the 3.8 will be fine in his truck. he will only have alittle less power but not really noticable in the truck unless he i loaded heavy but it still will be a good engine for him