I know it sounds kind of strange but I am trying to figure out the diameter my header pipes. I have triad headers with a triad muffler and I do not know if they are 1 1/2" or 1 5/8". I want 1 5/8". Is the measurement taken at the id widest point where the header meets the head or do I measure the inner diameter of the round pipe (after the stub pipes)? If mine are 1 1/2 then I will purchase a new set from tangerine.

I'm not sure if there is an industry standard for headers. But... Pipe is measured from the OD, conduit is measured from the ID.

Measurement is OD of the round primary tubes.

Pipe is measured on the ID.
Tubing is measured on the OD.

Pipe is rigid; it is measured by the outside diameter.

Hose is flexible; it is measured by the inside diameter.

Simple procedure :
99% of U.S.A. aftermarket headers are built from .060" (16) gauge flash-control steel tubing. This will equate to aporox. 1/16"of the tube wall thickness...or just subtract 1/8" (2× gauge) from your od measurement = id measurement. Sometimes just measuring the id inside the port flange is not accurate, as in the case if there was flow porting performed there. If you happen into the knock off ebay headers made outside of the U.S.A., these are mostly made from thinner 18 ga. mysterious alloy, and the above formula does not apply.
Or just call Triad and ask .
Marty