For your own safety and liability I strongly urge you to seek the advice an experienced, licensed structural engineer.
Members of Yahoo Answers are not qualified to give advice on safety-critical issues, particularly in the realm of lifting heavy structural loads!
If your welds fail, and equipment is damaged, or someone is hurt or killed, what are you going to say?
"I was just following the advice of a bunch of strangers from the internet....." For your sake I hope not!
Call an engineering consulting firm right away.
I can tell you this much: MIG welding is to be avoided, since most welders do not have the knowledge required to make sound MIG welds on heavy structural items. MIG welding should only be used for thin materials under 3mm. Unless you have EXTENSIVE experience wiuth MIg, in which case you would not be asking these questions.