was it from your heat sheild? i jsut had to weld up a hole bout that big, pre-cat also, from my heatsheild vibrating itslef loose.
but luckily we got a mig welder at my shop, so it was no prob. but they make this kind of clay stuff that you knead together and put it over the hole and let it sit for couple hours, i think it is called exhuast leak sealer.
It was on the "downpipe" after the exhaust manifold. I do think kit was from the crusty old heatshield. I fixed it with this exhaust bandage thingy I got from advanced auto. it looks like crap but it seems to work fine. When i put the heat Sheild back on it'll be hardly noticeable. Guess its time to get an exhaust lol.