Get a U-Bend in the size desired.

Cut it in the U section so that you end up with a narrow and pointed U-Bend when you weld them together.
(About 3" - 5" or so will be removed from the middle of the bend.)

Cut this narrow pointed U-Bend at the top end at the right place so it matches the diameter of your head pipe.
Either weld the head pipe to the U-Bend or weld a short run of pipe there which will make the U-Bend a Y pipe.

Done right it will flow fairly well.