Strip off all the soundsproof/sealant....front front to back identify all the points where the chassis fixes to the body (usually a 'U' section chassis spot welded to the body) and work down each (seam) stiching (welding)the lip of the chassis to the body...differing opinions onthe next part...weld the two together leave a gap then weld again down each seam....we usually weld approx 25mm leave approx 50mm....mark it up before you start to make it uniform...

HTH

Regards

Ian