318's didn't get a 4-barrel intake until the late 80's. If you go to some swap meets you should be able to find a factory 4-barrel 360 intake very cheap. I found one on a $1 table at a swap meet once.
its a 73 340 intake that come off my challenger i could send you a pic if you like i was looking to get maybe $50 for it but it does weigh 45lbs so i dont know how much the shipping would be ....