Lisa,
If you don't mind the weight, you can glue a thin piece of furniture-grade plywood to your stretchers and then glue the linen to it. I've found good success in doing that with smaller pieces, but I'm not sure how the weight would be for 30 x 40. You could try getting the local lumber place to custom cut you some out of 1/4 inch to see (you might need thicker for this size canvas). You will need to add some bracing.
Here are a few links that I have found helpful:
http://www.ampersandart.com/featured...ed-artist.html
http://www.amien.org/forums
I'm working my way up to the larger sizes (only done to 16 x 20 so far), so I haven't tackled a 30 x 40 inch canvas - yet. I feel it is helping my skill level to practice first. My first attempt was pitiful, but the improvement was rapid. You also might want to try your hand at some smaller sizes to get a feel for the task before doing this large a canvas.
For my 16 x 20 inch canvas, I glued my plywood panel to my stretchers. I used Golden's GAC100 to seal the plywood. I also made sure that my pre-primed linen was smooth on the back - I found that if I didn't use something like sandpaper to smooth off the nubs, they would show through on the front. I then applied Lineco archival glue to the panel, flipped it over and positioned it on the linen, turned it back so that the linen was facing upright and used a 6 inch wide brayer to get the linen smooth. I then weighed it down with a board and heavy weights and let it dry overnight. It came out very nice - the surface was perfectly flat: I folded and stapled the linen around the sides of the stretchers like I would normally do with a canvas.
For the Dibond, you would need to research the right type of primer to use. I thought I had that info, but can't lay my hand on it at the moment.
If you need to get a piece of Dibond quickly, I have two 36 x 48 pieces and I will be happy to sell one to you for what I paid. I'm about an hour north of Atlanta.
Hope this helps.