Well you have a rather LARGE list of questions I will attempt to respond to from my experience.
1. On the firewall flange
The wraparound or boot cowl as it is sometimes refered to is riveted to the firewall flange. The top and bottom engine cowls are secured to the firewall flange/wraparound with a combination of screws and fasteners, depending who has 'improved' upon the original design.
2. Still would like to know
If the windscreen is for a 7ECA, I am confident it will fit BTDT. The difference is the shape is more streamlined rather than the bubble appearance of the original. YOU DO HAVE TO FIT IT regardless of the type you have or buy and a 1/4' router bit in a die grinder serves as a suitable trimming tool.
3. Need to find two throttle knobs
I have a bag full I bought off of Ebay (user supplied parts section

). Let me know your mailing address and I will post them to you.
4. The 3-1619 Tail Wheel Arm
Yes the Wag Aero part will work. I also have several rudder frames and some steering horns. I don't recall the shank on the steering horns at this time. 419 three 0 five 36 seven 6
5. What do most people do for the panel/dash cover?
I replaced my instrument panel with one from Wag Aero. Original was looking like swiss cheese. I didn't have any cardboard anywhere that I recall.
I re-read your question and if you are talking about the part of the wraparound between the dash and the windscreen, I covered the metal with Stewarts Eko-bond and then laid black fabric over it. The fabric was then secured with a small sealing iron at 225 degrees.
6. My Champ was a 7FC converted to a 7EC
I am speculating (guessing) because the 7ECA I owned had rods between the front and rear rudder pedals and the 7CCM has cables, so talk with your IA and see what he thinks. He is the one who has to approve the aircraft for conforming to the TCDS and approving it for continued airworthiness.
7. I am thinking of retaining the generator and starter
Not a problem at all and you will definitely save dollars, but carry a few extra pounds. The lightweight alternator and starter are attractive when you are nose heavy.
Now I feel like I've finished Chapter 1. Good luck and hopefully Paul A will correct my observations.