RE: differences between the 4.2 an 3.8
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
98 3.8L 11psi P1SC Procharger,97 junkyard engine ,3.73`s,7.5 Detroit Truetrac,TA rear girdle,B&M shift kit,2500 stall
mods to complete
8.5:1 ross .030" pistons,Eagle H-Beam 351W rods,Comp cam 224/236 .569"/.578" 112* lobe sep,Rhodes variable duration roller lifters,Pro-M Univer MAF,siemens deka 60lb injectors, upgrade to D1@20psi,Delk Main Girdle and Main studs, Head studs,Comp 1.7 roller rockers, home ported heads CFM 216/196,PI 3500 stall