If you're experienced with a Dremel it is pretty easy overall but it takes a lot of patience and work. I just did it to my chassis, I don't have any pics, but I used two bits mainly - the standard metal cut off wheel and a high speed cutter.
I used the cut off wheel to go down the center of the chassis's rib and seperate the two halves of the center section. One cut went right down the middle, and another cut went parallel to the bottom of the chassis into the middle.
Then I just used the HS cutter to do the rest. I got the bulk of the material out of the way in a few passes then went back through it and finished it up using the same bit. Once I had the center section cut in half and the material out I spent a lot of time cutting the ribs down flush to the deck. You can tell major cutting was done to the chassis as I didn't go through and polish it up, but there is no evidence of the ribs cast in the tub anymore.
Just the center section took around 3 hours total. Once I had it all cut out I took a rounded grinding bit and smoothed it out a bit so it looked nicer and chamfered the edges.
This is what I mean by high speed cutter:
Not a great HOWTO, but I hope that helps some.