5.2 Magnum exhaust manifolds on a Valiant?

Has anyone used 5.2 magnum manifolds on a A body 1969? How much better do they flow than the old 340 ones? I can get them for $15 a side. I have 1972 340 E body drivers side and a ebody ? pass. Is it worth the differance? Or are headers the way to go?
1969 Valiant ,318,4bl,cam,4.10's, 4speed, lakewood bell,
(still in planning stage)
Thanks.