I see no proof that Cooper had any knowledge of aviation engineering whatsoever. Its much more likely he was a novice who had just a basic understanding of aerodynamics and aircraft operation. Picking the 727 as the aircraft of choice is a no brainer. Anybody with a triple digit I.Q. would make the same selection because of the aft stairs. That selection would require no more effort than spending an hour at an airport, watching planes on the tarmac. The fact the CIA was also using 727s is merely a coincidence.
As for tying the money around his waste, how many people would come up with the same solution if they were presented with that problem? 9 out of 10? Where else would he attach the money? His feet? His neck?
There's no proof that Cooper lived or died, but consider this. The newly released FBI transcripts show the pilots didn't know where they were when Cooper jumped, but thought they were in the vicinity of Portland's suburbs. The discovered ransom money was found on/in the bank of a very large river, and showed no signs of being handled since the jump. Finally, after 40 plus years, no one has produced any compelling evidence that Cooper survived. You would think that any of the dozen or so "suspects" that were alive after the jump would have kept and shared some type of souvenir something. After all, this was a very high profile heist, not the kind of theft someone would choose if they did want attention.
A number of airliners in the early 1970s had aft stairs. However, the 727 was unusual in that those stairs could be unlocked and lowered to some extent in flight. This piece of information was not widely known even by Boeing personnel. When Cooper purchased his ticket in Portland, he asked the ticket agent if the incoming airliner (which had not yet arrived) was a 727 and was told that it was. So Cooper was specifically looking for a 727.
In all probability, it was the CIA or one of its front organizations that paid for the engineering and flight tests that Boeing did to determine the effect of lowering the aft stairs in flight on the flight characteristics of the 727.
Your remark about the airliner cockpit crew not knowing their exact location when Cooper jumped is interesting and very probably completely accurate. If the airliner had been on the centerline of V-23, or any other airway, all the crew would have to do is glance at their VOR and DME readouts and they would have an excellent idea of their location. However, Rataczak has publicly stated that when Cooper jumped (or they thought he had jumped) he made a radio call to the ATC people to "mark your maps".
The above means to me that the airliner was not on the centerline of V-23, or even close to it, when Cooper jumped. In all probability, the airliner was off-airway and being vectored by radar by ATC in order to bypass Portland on the west side at the time Cooper jumped.
Also, when Tina states that Cooper was apparently familiar with parachutes, because he put one on with relative ease, it should be remembered that Tina may have never seen anyone put on a parachute before.
Nevertheless, Cooper knew how to put on a parachute and knew that the aft stairs on the 727 could be lowered in flight. This means that he had at least some aviation experience and was familiar with the end results of the Boeing/CIA tests related to the 727 aft stairs.
Let me add that tying a 22+ pound money bag around his waist is not a very good idea. The parachute opening shock could put a 200+ pound impulse load on his spine and that would probably do some damage. Cooper should have tied the money bag to the parachute harness.