Lamda sensor mounting

[Ade + Lamb Chop]

Hi,

I am running a Mini with a highly tuned A series engine (100bhp from a
1.3litre). It has a carb and Distributer but I am thinking about adding a
Lamda sensor with a dash mounted gauge so that I can Keep an eye on the
mixture.

Where in the exhaust system is the best place to mount the sensor? Close to
the head? 6ft down the exhaust? I have a Long center branch exhaust
manifold which doesn't bring the ports together for about 3ft after the
cylinder head. The manifold is also new (and expensive) so I would prefur
to mount is in the exhaust if it doesn't affect the reading.

I plan to get a sensor from a car in a scrap yard are there any UK (comon)
makes of cars that have better Lamda sensor than others?

In the UK they recently banned Leaded petrol. They made Lead replacement
petrol available to 'replace' it. Does LRP clog lamba sensors like Leaded
petrol does? I can run Unleaded but LRP has Higher Octane rating than
Unleaded and is cheaper than Super Unleaded.

Thanks,

Ade