what year/transmission do you have already? which v6 do you want? are you 2wd? that'll all factor into what it costs. Honestly though I think your best bet would be to buy another truck with a v6 in it already, or go all-out and do a V8.
420 pounds is nothing compared to the mass of the 302 small block. Although it's a compact V8, compared the the Chevy 489 Camaro down the street from me, it's still a heavy beast. I bet it weighs around 500 pounds without accessories!