best thing to get the welds off are power drill w/cutting off tool attachment with a steady hand.
With the suspension kit I would take a look at
www.mustangdepot.com 's suspension kit, I think it comes with more choices and is pretty reasonably priced. If youre getting new leaf springs go with 4 or 4 1/2 leaf. They will probably be right around what youre looking for. I'm putting 5 leaf race springs in my 66.