What you have there is normal "aging" on a C3 even the C2 cars do the same thing.
It is far more prone to show up on earlier C3s than later ones made from SMC.
It is just seam separation do to age. Not a crack, like out in the middle of a
hood or so forth. Drilling a hole at each end will not benefit much here.
If you feel the backside of your fenders in the location of the seam you'll
notice a bonding strip about 2" wide. You'll have to cut "A Lot" to go through.
So how deep do you need to go? Deep enough to apply 3 layers of 1-1/2 oz.
mat and SMC resin (I believe your car is a '78?). Then you'll never have to
worry about those lines ever showing up again.
Think of it as your turning the 3 piece front end into a 1 piece unit.
Here's an example of what I did on a '72 years ago.
After the layed up glass was ground to shape and ready for filler.
Filler applied.
