Fill a large pot with water and bring to a rolling boil. Stir in salt and pasta and return to a boil. Cook pasta uncovered, stirring occasionally, until just shy of firm to the bite ('al dente'), about 6 minutes. Reserve 1 cup pasta water; drain pasta and keep warm.
Meanwhile, heat olive oil in a large skillet over medium heat. Add garlic and red pepper flakes. Cook gently until fragrant, about 45 seconds.
Add the clam juice. Let it simmer until reduced slightly, 4 to 5 minutes. Stir in marinara; simmer for 3 to 4 minutes.
Stir in clams and heat gently for 1 to 2 minutes. Add pasta directly to clam sauce. Toss continuously.
Add splashes of reserved pasta water while tossing until the sauce becomes glossy and lightly coats the noodles.
Turn off heat. Add butter and parsley. Toss until butter is melted. Taste; adjust seasoning with salt, pepper, and lemon juice if needed.
