Behold, mike signalled from keyed 4-1000A filament:


1) Does Radio Shack sell torroidial cores?

No. Go to http://www.amidoncorp.com

2) Would I wind it similarly to the air core in that each winding is
separated by one wire width and tapped every 3rd turn?

No. Your winding will be determined by the mu and Al value of the core.
Amidon supplies those values and the math.

If I understand the concept correctly, it's seems the PI network tuner
tunes both ends of the equation. This would be good as I am using a Sony
ICF-SW7600GR and dont have a clue what the input impedance might be.

PI network is great if both the source and load impedances are unknown.

3) Is that how it really works? Tuning both the antenna and the line out
to the receiver?

It' an impedance changer with low pass filter effect as well. Nothing
more, nothing less.

Arnie says this tuner will work down to 5kHz.

Not unless your inductance in in the hundreds of mH and you have uF of
capacitance.

And given my research of
this tuner type, I would need to switch in extra capacitance to get down
low for say the AM band. I am considering adding in a few more toggles
to achieve this. My variables are 365pf.

AM band is easy to reach. Wind your coil for the lowest frequency you
expect to receive.

4)How much extra capacitance would be needed? And would this be
practical or should I switch in another inductor creating a PI-L?

PI-L has a useless advantage for receiving, only good for transmitting.

I suggest you go to your library and read an older (mid 80's) ARRL Radio
Amateur's Handbook. They have a lot of vanluable information on this
tiopic, with the related math, even charts for Amidon toroids.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca