Let me say this upfront, this is for the 2000-2002 bumper and trunk mounts... i.e. Gen1 Non-LSE (I don't know the dimensions of '03-'06, which already have cutouts for single exhaust tips).
Basically there is a real bumper under the bumper cover.
There is the valence on the bottom, and then a curved part of the bumper cover just above the valence.
The "bumper" sits about 1/2 way into the curved part of the painted bumper cover. Stay
below that 1/2 way mark.
I put about 1 foot wide blue painters tape on top of that imaginary line to create a level horizontal line. Step back and eye-ball it good, or use a small level on the painters tape. I just eyeballed it and kept moving the tape, and measured up from the valence on each end of the tape. Once level, I put another tape butted up under it horizontally. That then is my horizontal cut line, between the tapes.
Do the same to the other side. Step back and check both sides and adjust.
Tape off everything below that.
Measure your dual exhaust tip widths. I was going to leave 1/4" all around. Dimensions say it's 6.4" wide. So I was going to cut 7" wide.
Mine actually measured 6.25". Plus .25", plus .25" = 6.75" wide cut. Let's call this the "Wide Cut". (Measure your width, add .5" inches).
Start underneath the car at the valence and follow Pektel's instructions to mark a line straight up the original valence cut and up to the painted bumper cover:
http://www.lincolnvscadillac.com/showthread.php?t=44774
(Make sure the vertical line does not lean towards the center of the car, to stay away from the trunk mounts... because you will be going 6.75"-7" on your cut towards center). Make sure that line is vertical.
Measure from that vertical line where it meets the valence and measure over the distance for your "Wide Cut". Mark a line straight down the valence to the very bottom. Check both vertical lines of your "Wide Cut" and make sure they are really vertical. Measure your width again, and again.
Find something like a plastic lid, or draw a 3.5" circle and cut it out to use as your template to make the upper curves (3" exhaust tip, plus .25" on each side would be a 3.5" circle).
Draw your first half circle starting at top of valence on one of your vertical lines, and up to where the two horizontal tape lines meet.
Go to the other side of your "Wide Cut" and draw another half circle using your 3.5" circle template.
Do the other side of the bumper the same.
Stand back and triple triple check everything via eyeball, measure again and again. Get it right. Don't go too wide on your cutting (you can cut it wider again or sand it or use a file, or a dremel sanding disk etc to make it wider).
Basically don't cut too high, don't cut too wide.
Get your coping saw (mine was adjustable so you can rotate the blade), use a fine tooth blade, and slowly and carefully start sawing. It cuts/saws easy. Keep your blade square and saw. Rotate the blade during your curve cuts, and get it to 90 degrees for the horizontal cuts. (This keeps your mounting arm of the coping saw below the bumper). Cut slow, follow your pencil lines.
I used 150-250 grit sandpaper held around a very small board to smooth things out. Be sure to tape things off again before sanding so as to not scratch the paint on the bumper cover - this is important.
Use color matching paint and/or flat black if needed on your fresh cuts to make them go away (the cut area will reflect somewhat off your shiney exhaust tips).
It doesn't look bad at all without paint on your fresh cuts.
Reference the Pektel link above, just measure out your "Wide Cut" correctly, tape it off, use a pencil and draw on the tape... re-measure, eyeball it, then cut it out.
In this picture below where I drew a "RED" line is the "trunk mount" I am referring too... don't let your "Wide Cut" encroach/intrude towards that. Just start far left or right to the outsides of the valence/bumper cover when starting your initial measuring.
The exhaust tip picture below has red lines showing where the "trunk mount" will be very close to.
The Bassani #4820 muffler is 4" deep, 8" wide and 16" long. You will need a muffler within those dimensions or less, to get the tips high enough off the ground to tuck under/within the valence, to get the tips away from the trunk mount, and 16" or shorter so the tips don't stick out too far past the bumper. We actually angled the Bassani muffler diagonally up from the axle to the bumper so it follows the body line... you can't see it sticking under the bumper from the side of the car.
Last picture shows where to measure your exhaust tips for the "Wide Cut". Add .5" to give a .25" cutout border around the tips... (or just a bit more than .5" like .75"). I will probably be at .75" when done, to get another 1/8" more border.
(I am sure this can be said shorter, but it's the best I can do tonight after zero hours sleep last night).
