I am guessing that there are a number of bike tourists out there that have used Amtrak boxes on airlines (either full size or cut down) and that is why I asked for their knowledge, Hopefully there is no problem flying with a full size Amtrak box. If so, it will be very useful to me.
As I noted in my original post I will cut the box down as a last resort and I will plan on arriving at the airport with enough time to do that. Not ideal for a number of reasons including that my wheelbase is 67" and the length of an Amtrak box is 71" so reducing the length by 5" is a squeeze. I guess I could let the air out of my tires and still not pull the front wheel.
It's been 40+ years since I flew with my bicycle and at that time it was without a box - remove the pedals, drop the handlebar stem and seat onto the frame, remove the wheels, remove the skewers from the hubs and tape the wheels onto either side of the main triangle of the frame. Probably I removed my chain and rear derailleur as well or maybe I just taped them up, I don't remember.. Then hand your bike unboxed to the agent at the check-in counter. It's much nicer to use an Amtrak box where you just remove the pedals and lower and twist the handlebars.
The trips I have done recently have been on Amtrak and usually without a box but I have used their boxes twice. They are $15 and are a very high quality heavy grade of cardboard. Better boxes than what I have seen new bicycles shipped in. At the end of a tour when you are in a city unfamiliar to you it seems like a great place to get a box and that is why I am guessing a lot of bike tourists have already flown with Amtrak boxes.