[dsraven]

grab a piece of copper pipe, flatten it, use it as a backer to fill the holes. the weld won't fall through then. big holes need a plug of metal to weld in. grab some hole cut outs of a size bigger than your biggest hole. drill the holes (with a step drill so they are round) to the size required for the plugs, then all the holes can be easily plugged. try welders series for some unholes (plugs) and lots of other hotrod requirements. they are Canadian so the USD goes a long way. lots of interesting stuff on their website too. some plugs from electrical boxes will work as well but they are galanizes so you takle more time grinding that off than it is worth.
to keep the copper in place I have a C clamp with the bottom of the C cut off, then a magnet attached there. you can stick the magnet to the project, then bear down on the copper backer with the screw part of the c clamp. works pretty good especially if the c clamp is a wide one so the magnet part is further away from the weld, to stop the spatter induced by the magnetism.