Exhaust fumes in the cab

[nicktheknife]

All right, I am getting some serious exhaust fumes in the cab, its to the point where I shouldnt be driving it, by the time I get home I am nausiated and feel like I am going to pass out. My question is, the exhaust ports on my headrs are rectangle, but the holes on my headers are oval, should I use a gasket that is rectangle, oval, or both? Also, where would it enter the cab? One guess is through the holes on the heater core housing where the two hoses go into it, theres a gap, but does that go directly into the heater duct? Should I plug those holes up with some type of sealant? Please help.