You can keep the frame in tact, and weld an extension directly to the old head tube , Or remember to cut the top tube out AFTER the new lower cross tube is welded in place.
Use the old head tube to hold the new longer head tube in alignment while welding, by cutting the old head tube in half and clamping the new one to it.
The less you have to cut away the easier it is to keep the frame straight. See more info here.
1 x 2 inch 4130 Rectangular Steel Tubing is the best thing you could possibly use to for the bottom tubes on the long bikes.