This is some interesting info on the 429/460. I didn't know someone made an aluminum block weighing just 145 lbs.
With aluminum intake, heads and an aluminum block, total weight should not be much more than an all-iron 351. I figure these items would save about 200-250 lbs. in all.
For a Cobra, you could have your cake and eat it too!
http://members.tripod.com/lyc_42/fordv8/bb/bb.htm