[TR] Lucas Master Battery Switch - SSB103 - and other grounding questions

[Carl TR]

Good morning all - I hope your weekend projects are going as well as mine...

I have an 'earth' side master switch I want to install on my project for a 
combination safety and theft prevention.  see: 
http://www.autoelectricsupplies.co.uk/product/144

My plan was to place it on the underside of the dash support bar (steering 
column side) by cutting a hole in upper right (driver's) side of the battery 
box, using a grommet (probably the same one as the wiper) for insulation 
purposes.   Also, I was going to put some heat-shrink insulation on it from 
just before the hole to the connection on the switch.    To make it look 
original (and to hide the fact that there is a switch) - I am going to 
somehow connect a fake end to the firewall above the battery, feeding it 
from the heat shrink portion of the ground cable.

I need to locate a good spot to attach to ground after the switch that won't 
create havoc to instruments, etc under the dash.   Any suggestions?  Pedal 
'box' mounting bolts?

Also, a side affect I hadn't planned on with having both the body tub & 
chassis powder coated - any suggestions on how to insure a good solid 
ground.   Would attaching a  cable (similar to the engine to chassis ground) 
from the tub to the chassis  in a couple places suffice?  Perhaps one at the 
firewall and the other at the back somewhere.  I'd need to grind to metal 
for the connection.

Finally, one of those inquiring mind kind of questions.   The concept of 
electrical current is that there is a circuit coming from the battery to the 
load and back through the 'ground' to the battery to complete the circle. 
If a person touches both sides in a house circuit - you definitely feel it. 
However, if the car metal (chassis/body) is the grounding side - why isn't 
there an issue?  Or am I thinking it through wrong.

As always - thanks for the responses.

Carl