If you can put use a thicker steel for your plate (2mm shouls be plenty), and put it under the hole. Then weld down trough the good steel around the hole onto it. This is the easyest way of welding, using gravity to your advantage. By using a thicker steel for the plate you can mely right through the original metal onto the plate, without melting that too.
Remember welding is all about the preperation. Make sure you've got all the rot (and paint etc) out of the way befor you start. It's very easy to think the hole you can see is all the rot there is. When you start welding you'll soon discover that the 'thin-ness' of the metal goes a long way from the visible hole.
As for the ribs, hammer them flat to the plate as you go. They'll lay down a lot easier once hot from welding nearby.
You'll only need a low power, and farly slow wire speed (asuming your using MIG) for this job.
If you cant get a plate under the hole, you'll have to put one over the top. Doing things this way use a thiner gage plate (1mm maybee). Weld the edges of the patch to the original metal, using the welding 'gun' at an agle so the wire is hiting the edge of the plate and the original metal at the same time. This will make it less likely that you'll blow holes in the original metal.