I read about the use of yellowish glue on some parts such as the driver side trunk pinch weld protector guard. My car is missing that piece but I do have one from a 65-66. Firstly is that guard black?
Typically no not for your application and 68's are not shaped like 65-66's in many cases - you don't want the style with the V's or notches
Second what sort or type of glue leaves the yellow residue?
Same product as used to install door and trunk lid weatherstrip. You will find some modern suggestions if you use the search feature. Buy only a small amount as suggested
PS - Splitting this question from the thread so the question and information can be easier found for others looking for the same