Hey Henry how’s it going?
My 68 had a couple of the stud mounts broken and one of the tabs gone .
I used JB weld to repair stud extensions at the front mounts and made a tab from aluminum and more JB to fasten it . I roughed up the surfaces so the JB would have a good bond.
I had to be very careful not to over tighten the assembly to the fender, so far it has held up real well.
Hope this helps.

I was able to repair a crack on my 69 headlight door (fender extension on 69s) using a tig welder.

JBWeld has always been my go-to for epoxy, but I recently came across a product that is better for most cases. It is called Marine-Tex gray. It is similar to JBWeld, but is more of a formable, putty consistency. It is also about 3x the price.

As for fixing the pot metal Mustang parts either will do the job.

My experience shows me that using these techniques will help you make a permanent, stable repair.
1. Rough up the metal with a 40 grit flapper wheel so the epoxy can get a good grip.
2. Glue to the largest surface area you can apply to. The more surface area the glue is applied to, the stronger the repair will be.  This usually means you need another piece of aluminum to span across the broken part. That part myst also be roughed up with 40 grit.  Application of the epoxy to 100% of the surface area of the overlapped parts will result in the strongest fix and will keep future corrosion away.  Some thicker parts containing enough surface area where the epoxy will be applied might not need an additional piece.
3.  If at all possible, bolt the part in position before the epoxy hardens so it is not stressed by bending to fit if installed after hardening. With JBWeld, masking tape is a big help to keep the epoxy from running before curing. It peels right off after drying.  This is one big advantage of the Marine-Tex. It is a putty consistency and does not shift once applied. Bolting in place is very important on headlight doors as their alignment is typically a compromise at best.
4.  I sometimes use pop rivets to help hold things in alignment. California Pony Cars makes a great fix piece for the 65-66 headlight doors. When attached using all four of these suggestions, they are better than factory.

This is not to encourage ANY purchase from CPC. Some of you remember they stole my cable clutch and tie rod adapter designs from me several years ago. Purchase of parts from them or any company that buys wholesale from them is supporting a stolen design. Any parts that look like MustangSteve parts can only be purchased from www.mustangsteve.com or the ebay store mustangsteve66.

Bearing Bob wrote:

JB Weld has a really good product called Steel Stick. It's a two part putty like material that you knead together. I've had good results using it.

I've also used this, and I was thinking the same thing when I read about wanting a putty consistency.

Make a metal patch out of 18 or 20 gauge  then rivet or jb weld