There is a much easier, and I believe better, way to cut the exhaust pass through. It was posted here almost 10 years ago by Jeff Scoville, who isn't active on the forum any longer. Here is what Jeff wrote at the time, slightly edited because of context:
A good quality (Greenlee) holesaw will work to cut the passthrough.
As a bit of advice, what I do when trying to run a holesaw through on an angle (which is what you'll need to do) rather than cutting a hole through each side, then ovaling them out and hoping your in the ballpark, what I have done is get a good quality spotter bit that fits your mandrel.
The spotter bits are usually about 12" long. You center the bit to where you want your exhaust to run through and the drill bit should have enough length to go through both sides of the X-brace. Now here's where you need patience, just let the holesaw SLOWLY chew it's way through, it will want to bind if you don't go REAL slow.
You can run it through all the way from one side, or once you've cut one side go at it from the other side so as to keep an eye on the holesaw bit.
Lastly (i'm out of breath) if you chose a holesaw to fit a given pipe dia. larger than what your exhaust will be, you can run a piece thru and let the bodyshop weld and trim it giving you a nice "factory" looking hole.
Whew.
Hope that wasn't too much info, just trying to be clear.
Have fun!
I did it this way and it worked extremely well. It was a PITA because I probably didn't go slow enough and the saw frequently did bind and the drill kicked back, breaking the drill bit. Still, it left me with a pretty clean hole through the cross member that looked pretty stock and was a breeze to fill in with the exhaust pipe for the "nice "factory" looking hole" that Jeff promised.
I do have a couple of suggestions to add to what Jeff gave me. First, the 12" bits bits are expensive and I went through two of them before it suddenly hit me that once the hole was drilled I was only using the bit as a guide for the saw. So, I switched to 1/4 rod which worked great. I bent a few of them along the way, but at about 50 cents each it was no big deal. It is also easier to drill the initial 'pilot' hole without the hole saw attached to the drill. Just use the long bit to drill all the way through both sides, then use the rod in the hole saw to keep the saw aligned.
Second, use the best, deepest hole saw you can get. Cutting at an angle causes the saw to bottom out pretty easily and I had to do a lot of ancillary cutting and grinding to get all the way through. Besides being difficult - and I did this on my back with the car on jackstands - the final cut wasn't as clean as I would have liked and it took some real effort to make it look professional. I think a deeper saw would have made a difference and, if I had it to do over, I would have bought the best quality one I could find, rather than the standard Home Depot saw I used.
Finally, you might want to consider using 3" conduit, if you can get it, rather than 3" exhaust pipe. I recommend this for two reasons. First, I think it is a little heavier gauge then the exhaust. Second, it is a little larger - 3.25" which will help when routing the exhaust. I only happened on this by accident because there was an electrical crew working in my office at the time I was doing this and I had one of the electricians cut me a piece of 3" conduit. I didn't know that it was larger until I got it home, but since I'd already cut the 3" hole, I had to scrap it and buy a piece of exhaust pipe. I'd definitely go with the conduit if I ever do this again, but be sure to buy the conduit first so that you can match it's OD to the hole saw.
As Jeff said, Have Fun!
