Topband: Pse help: SHUNT FED TOWER+CAPACITIVE HAT

[Sante - IK0HBN]

Hi topbander fellows,
first I wish thank all them helped me on last november when I was 
trying to put a tower with a Yagi for 40 and a 4square array for 80.

Sadly the hazer which should have raisen the rotator+yagi showed a 
bad functioning, so it has been removed, with a lot of wasted money 
(to do and undo it).

To save what I can, I have reduced plan to ONLY 160 meter band and a 
support for 4 wire square on 80 meter.

>Beside the mechanical constraint which prevented me of using the 
>same tower for 3 bands (160-80-40), Dave WX7G kindly advised me that 
>probably interaction could have been with those 3 bands.

So, I read and read the Bible (ON4UN book) and bitterly I discovered 
that a YAgi for 40 meter at 30 meter high should have shown a deep 
null within 2 main lobe, so 40 meter band was definitively out!

Now I wish to use the pole as described before. Below its measures:

High: m. 28,7 out of the ground.
Diameter at base : 925 mm.
diameter at top : 240.

It's clear that pole is too short electrically speaking, so I could 
add an extension to gain 4,5 or 6 meter, but I still use a capacitive 
hat as GAP VOYAGER does. That extension will further help me in 
getting an higher support for 80 4sq arrays.

 From the pole, always hanging on insulated guys a 4 square array 
with Comtek box.
The verticals should reach the ground at half a meter high, so to 
deploy the radials system directly on the ground.

MY QUESTIONS:
How much should I extend the existing pole?
How much big the capacitive hat ( I thought at something like GAP but 
with hot dipped steel to assure mechanic resistance)?
Shunt fed: at which height/distance  put the arm for shunt fed wire?
May I connect the 160m ground radial to those for 80 meter? 
Everything should be buried?
Last but not least: It should be a definitive setting!

Thanks in advance, as promise I will summarize.

Hat off to the huge signals heard on last we on topband during 
contest. As a part time swl I used a short vertical for hearring you!
73's Sante