On restored cars I think we have a number of things going on.
1- Often the fenders are bowed to match the deformed (curved) hoods so there is often a very large gap left between the fenders and inner fenders. Even witnessed one "restorer" (using the term loosely
that used the expandable foam to fill the gap
2- But there was often allot more used when the Shelby front ends were installed and a lot sloppier than the regular Mustang applications generally - looking at pictures of original cars
3- We see allot of people using the light gray sealant that IMHO is not historically correct. A dark gray or black would be more correct IMHO. I would likely try some 3M cord chalking since it's easy to work with. Multiple strands can be formed together and I think you could mix the black and some of the light gray together to get a closer color. The problem with the stuff it that it will stay tacky for a long time. Original stuff likely was the same and took many years to dry as hard as a rock
Sure a couple of others here will have some excellent input and help