In the process of redoing the kitchen and we will be putting down new cork flooring in that area. We've opened up the doorway to the adjacent dining room by 5 feet and I will have to fabricate a new header/transition between the hardwood in the dinning room and the new cork. How to I make a nice strait cut inside the doorway to accommodate the new header/transition piece? Is there a special saw for this? The skill saw and a strait edge will only let me cut up to 2"-3" away from the casing...

I would probably use the skill saw and finish with a single carbide blade by hand (used to cut hardi board for tile) Or use my fein multi saw. to finish to the casings. even a utility knife and straight edge or chisel combo could possibly do the trick.

I would invest in a quality 3/4" chisel and pare it back from a scribe line. It's tough to mess up if the chisel is sharp. You may need to invest in a quality wetstone or a small fixture to set the angle and hone the blade on some fine sandpaper set on a glass plate.