2Rub off excess papery skin from garlic heads without separating cloves, and slice about ½ inch off the top. Place each head on a square of foil (cut-end up). Sprinkle each with 1 tablespoon of water and pinch the edges of the foil together to make a package. Roast until cloves are very soft, 40 to 50 minutes. Unwrap and let cool slightly. Squeeze cloves out of skins into a small bowl; set aside.
3Heat oil in a saucepan over low heat. Add shallot and cook until softened, 1 to 2 minutes. Add marjoram and crushed red pepper; cook until aromatic, about 30 seconds. Add wine and bring to a boil. Increase heat to medium-high and cook until mixture has reduced to ¼ cup, 10 to 14 minutes. (The wine will be nearly evaporated.) Reduce heat to low; add reserved clam juice and roasted garlic and whisk until smooth. Stir in clams and ¼ cup parsley; heat gently over low heat. Keep sauce warm.
4Cook pasta in a large pot of boiling salted water until just tender, 8 to 10 minutes or according to package directions.
5Meanwhile, make gremolata: combine ¼ cup parsley, lemon zest and garlic clove; mix well.
6Drain the pasta and transfer to a large, warm serving bowl. Stir lemon juice and butter into clam sauce, and pour over pasta. Toss gently, then sprinkle the gremolata over the top. Serve immediately, with lemon wedges.