Preheat oven to 350 degrees. Lightly butter and flour an 3-by-8-inch metal loaf pan. In a large bowl, using an electric mixer, beat butter and 1/2 cup sugar on high until light and fluffy, about 3 minutes. Beat in eggs, one at a time, until combined, scraping down bowl as needed. In a food processor, pulse almonds and 1/4 cup sugar until finely ground. Add flour and salt and pulse to combine. With mixer on low, beat almond mixture into butter mixture until combined. Beat in vanilla and lemon zest.
Transfer batter to pan and smooth top. Press grape halves into batter in rows and bake until a toothpick inserted in center comes out with a few moist crumbs attached, about 1 hour, rotating pan halfway through. Let cake cool in pan on a wire rack. To serve, remove cake from pan and slice.
Start by making rows of grapes on a short side and a long side of the pan. This will give you a guide to follow, ensuring an evenly spaced top.