273 to 360 exhaust

I'm trying to use the 273 manifolds on my 66 cuda. When I put the mufflers on
It leaks from the bottom of the drivers side. The engine is a 76 360 . I closed the smog holes in the heads. But I'm wondering if the 273 manifolds are too small fort the ports in the 360 heads. Maybe not as the right side isn't leaking. Only the left side. Any info would be appreciated.:banghead: