Not an answer to your question but....
My experience with replacing rear valence is they don't fit very well. Make sure you don't throw your original one away. Mustang Steve did a write up on getting the repops to fit better. Do your fitment and install it first before taking it somewhere for paint.

If your old one is nice it may be easier to just carefully cut the holes in it.

If you go with a new one, definitely follow mochaman’s advice and test fit it first. You will need to follow my method or it will appear 1/2” short on each corner.  Hard to explain how to do it in words, but it will fit perfect if you adjust the top flange so it forces the valance to be a straight line across the bottom eliminating some of the curvature.

TremendousWand wrote:

MS wrote:

If your old one is nice it may be easier to just carefully cut the holes in it.

If you go with a new one, definitely follow mochaman’s advice and test fit it first. You will need to follow my method or it will appear 1/2” short on each corner. Hard to explain how to do it in words, but it will fit perfect if you adjust the top flange so it forces the valance to be a straight line across the bottom eliminating some of the curvature.

Do you have a previous post on modifying an after market rear valance?  I searched and couldn't find anything.

thank you
 

Naaaaaaaaaaaaaaa....gotta come to the MSBB?? to get the  'COMPLETE instructions.
6sal6
 

If the rear valence you have is in good shape and fits well, you can search for a template to cut the holes - I remember seeing one Ford gave the dealers to do the retrofit.  Alternatively, you could borrow a repop and use it to trace out the holes.  If you're going to use a repop, you will likely find that it doesn't fit very well.  I have an original one w/out either backup lights or exhaust holes and decided to get a repop.  I had to weld on a bead to the ends a bit so it would not leave too large a gap (MS's procedure didn't work for me).  Your next decision is what tips to install.  The original ones are quite restrictive and the pipe dia. may be smaller than you're using.  This is what I used and they look great:  https://www.amazon.com/gp/product/B009R6ZTQ8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

MS wrote:

Trying to simplify my explanation...
If you held the valance up to the back of the car, in the orientation it would be installed, take the flange at the top and bend that flange so the top of it points more to the front. This is easiest if you place the flange in a vise and add more angle to the bend. Move the flange about 4” in the vise and continue until you have it bent all the way across. Bend it so the angle is exaggerated from what you think it will be when installed, but don't get carried away. Then when you tighten all the screws at the top, it will force the bottom of the valance to fit correctly. No bodywork is required. Be sure you have the correct mounting studs that attach the bottom corners to the car, along with their little clips that hold the studs to the valance. Sometimes a little percussion engineering to flatten the mounting bracket at the corners is needed if it physically cannot pull up tight enough. This can happen if a quarter panel or patch panel has been welded to the car.

This procedure works perfectly every time. What you are trying to do is get the valance to tuck up under the car further. Exaggerated, it would touch the gas tank. But it stops when the corners touch where they belong. There is never a reason to have to cut or weld anything.

You can easily verify this will work on your car. Measure the exact distance from the two lower points of the quarter panels where the valance is supposed to fit. Then measure the bottom of the valance, following the curvature of the lower edge. You will see the measurements are nearly the same. The balance might be 3/16” or so shorter so it will fit in the gap. The idea is to force the lower edge to take the shortest distance beyween the two points and not have a large bow in it.

I'm going to give this a shot.  I realize beggars can't be choosers but a picture would be worth a thousand words.

MS wrote:

I wish I had a valance to take pics of the mod. Bring it and your car to my place and we can get it fitted.
I’ll hammer while you shoot pics.

You are a very kind Fella to offer something like this Steve!
Big thumbs up to you