the 4.3 v-6 to a 305 swap if easier because a 4.3 is basically a small block chevy with the back 2 cylinder cut off. the 4.2 and the 4.6 dont have alot in common. you would ruin resale value and quite possible ruin your truck. open your hood and as yourself if you are willing to every thing you see. every wire , are the a/c systems inter changeable? are the radiators the same? this is the kind of project that would keep you up at night and you if you pull it of you get 25 h.p.
before you do it see how long it takes to change the plugs alone on the 4.6, it might change your mind