like notchbacklx said, getting a tubular k-member will make up for the weight gained, but comparing the power/weight, a 351 is worth it. Especially because of the amount of power it can handle. I'll gladly add 50 pounds to my car. But 50lbs is accurate as said before.
I looked into swapping over to a 351 and was going to do it, its not very expensive, but just out of my price range. Theres a lot of info on stangnet about 351's. Just remember you will have to upgrade all performance engine parts, because 302 performance parts on a 351 makes 302 power. You cant put afr 165's, gt40 intake, and an off the shelf cam and expect to make over 350hp.