That stock is a Romanian AKM stock and will not work with this build. The Yugo stick attaches slightly similar to a milled receiver fixed stock with a bolt that goes through the trunnion from the inside of the receiver toward the buttplate. To make the stock work you would need a "euro" stock block.
The plastic stock will probably not work either. You specifically need yugoslav furniture to fit this type of rifle because of its unique variation as opposed to most Eastern Block or Russian AKM designs.
And Cassini is right, the extra yugo latch is a bar that slides through the little dovetail track that retains the recoil spring. When the button is pushed, it moves a slotted portion of the latch into the right position in the dovetail to allow you to press the recoil spring forward to remove the cover. With the stock block you ordered, the recoil spring will hold the top cover just like an AKM so it will work fine.
ETA handguards in the picture won't work either. Google Ironwood Designs. They make great stuff. Or you can dig up a foreign yugo stock set.