exhaust valve dia. vs header tube dia.

as the ***le suggests, what is the relationship between the exhaust valve diameter and the exhaust header tube diameter? i`m going to ***emble a set of block hugger headers for a 270 c.i. dodge hemi, the exhaust valve diameter is 1.5". this is going to go into a street driven`40 ford coupe.
thanx

 

Go with 1.500" diameter tubes which would be great for low end torque...

 

Diameter is related to HP and rpm.
270 is pretty small.

1 1/2" should be plenty, maybe even 1 3/8"

 

how about port size ?