73 Engine Conv.
 

My cousin has a 1973 Mustang Grande with a straight 6 250. He was wondering what all had to be replaced if he converted to a 351?

I'm sure this isn't all, but - front springs (check canadian mustang, their midolyne bushings and hd 1" lowered springs work great on my 70!), radiator, don't know if auto trans's have diff bellhousings, but probably need heavier duty auto if auto, or likely bigger clutch if stick, maybe even heavier duty trans. Maybe even heavier rear end, at least if stick. Motor mounts and exhaust, and that should do most of it. You'd probably want to upgrade brakes, too.