Fill space in fitting?

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Welding front cross member into 1934 Ford frame which will also have a boxing plate welded in.  Gap at both sides between cross member and boxing plate is different width.  Need advice on how to close gap.  Four pieces of metal are coming together.  Two frame rails, one inside of the other, each is 10 gauge.  Boxing plate is 3/16" and will be just short of frame rail total height to provide a 90 degree step to weld into.  Need penetration into both frame rails.  Plan on step being equal to one frame rail thickness (10 gauge). Cross member is 1/8".  Should I fill the gap by putting filler steel between frame rails and boxing plates or weld boxing plates to rails, position cross member and fit filler plates between cross member and boxing plates?  If I fit filler plate between crossmember and boxing plate I have space for filler plate to be 1/4" bigger than crossmember and still not be bigger than boxing plate height.  Filler plate on one side is 10 gauge and 1/8" on the other.  I am concerned about welding the front cross member of a car to such thin steel.  Is it better to grind the cross member on both sides so they both will take 3/16" plate providing consistency and a thicker plate to weld to?  People's lives are at risk here.  I am new to welding.  Should I tack and have a pro do final welding?  Using Miller 185 MIG 75% argon 25% co2, .030 wire.  I am getting good penetration on the practice welds I am doing.  Testing by putting in vice and bending with a hammer.  Welds are not breaking.  Thanks  (tried to attach photos to show different approaches mocked up but was unable to)Last edited by ctkid343w; 02-28-2011 at 11:32 AM.
Reply:Not sure what your saying but I would cut my box plates so they are the same size as the opening you need to fill so there is no gap and weld it like a normal joint.
Reply:Thanks ironmangq.  I could grind the cross member so it would need the same size boxing plate on both sides, get new boxing plate material and cut them out.  Result is cleaner than my options but almost twice as heavy as boxing plates will be 3/8" the entire length rather than 3/16" the entire length and a little thicker around the cross member.  I like your option.
Reply:Another suggestion I received elsewhere was to squeeze the rails in to match the width of the crossmember & boxing plates.  Considering it.  Impact is about 3/16" on one side and 1/8" on the other.  Both are within 1934 tolerances and probably with 1974 Mustang II specifications.  I would probably grind crossmember so both sides are 3/16".  Any thoughts?
Reply:sounds like you should have someone that knows what they are doing to be doing the work . .. just by reading your original post your a new to welding and are trying to do structural modifications to a motor vehicle.....  if done wrong and fails you will endanger others on the road and the driver and passengersMiller Xmt 350Lincoln Ln-25Ahp 200xSmith Gas Mixer AR/HTig is my Kung FuThrowing down dimes and weaving aboutInstagram http://instagram.com/[email protected]
Reply:Originally Posted by ctkid343wPeople's lives are at risk here.  I am new to welding.