If this shield is for the same design motor mount as in the '65, the main reason I see for them is to keep dirt, gravel, etc. from accumulating in the empty space between the mount and the engine. I had the engine in my car ('65) and forgot the shields and went back and raised each side of the engine slightly with a floor jack to slip the shield in place.
Somebody sent me a picture of a heatshield (with ruler next to it) a few months back, and I went ahead and made my own set out of a scrap piece of stainless steel. If you have the tools and are just feeling like fabricating it yourself, it's fairly easy. But at $20 a set from NPD, you won't save a bunch. Let me know if you want the picture.
1966 GT Coupe (Tahoe Turquoise with black pony interior)