Look at a 70s Miyata catalogue, here's one possilbe source
http://www.velo-pages.com/main.php?g2_itemId=26669
I have a '72 hi ten Miyata, slightly better components than yours. I ride it routinely, it's a terrific bike and a solid commuter. However, they are heavy...and I had to change the rims to SUN M13 hooked rims to use high pressure tires.
That rack is a joke, get it off that bike! I opted for a Pletscher vintage style rack for a set permanently mounted panniers, but they're not intended to handle real touring weights.