very hard, everything is different
totally different trucks, totally different wheelbases
you name it, its different
for real..
i would get a hold of mudder 67 as he has a 67 body on a 78 or so chassis, but both are pickup ones, and even then he had to chop frame and change all body mount frame brackets a list of other things
if you attempt it then i can almost guarantee youll ruin two trucks
good luck