Is it made? 1970 Duster left frame repair cap

I don't know if you are looking for the front or rear frame rail, but if it is the rear, it looks like you need to order the full cap. The front can be bought in sections. Ironically, I am presently working on installing a cap on my Challenger. I ended up buying a full cap because the rear section offered for the front rail was not long enough to cover the area I needed to cap. So, I ended up cutting it down to make it work. Anyway, here are the part numbers links to what I found for your Duster.

Rear cap: MM8160 - 1960-76 Mopar A-Body Full Rear Frame Rail Repair Cap; LH

Rear section of front: MM8151 - 1960-76 Mopar A-Body Front Frame Rail Repair Cap (Rear Section); LH