A little update:

So many things differ between an I4 and V6 B5, as far as installing the V8 goes.

1) Mounts - new custom mounts made to integrate V8, different from V6 base car.
2) Subframe - tranny n/s bracket needs to be cut off, and V6 one welded in place.
3) Tranny n/s mount - being replaced with V6 bracket, to allow n/s catalyst to fit.
4) Airbox - 1,8T unit has intake facing front of engine, being replaced with V6 item.

Car is now running, for some reason though, the S8 immo would not code to the B5's key and immo box, so a different approach was needed.

It involved obtaining a B5 ECU, immo box and key from a junkyard. All came from a 1998 A4 2,8 quattro. The immo box was then plugged into the car, the transponder removed from the new key (was glued in, what a b4stard it was to remove). Finally, the immo processor was carefully desoldered from the 2,8 ECU, and soldered in place of the old one in the S8's ECU. Voila!

Had a scary moment last week - the fuel filter decided to spring a leak of major proportions. It was so rusty, I was surprised it held pressure up to that point. It looked like a cylindrical brick, and after replacement, the injectors had to be removed, and taken for an ultrasonic clean. The fuel lines were then flushed through, before replacing the injectors, and firing her up.

Another issue...the cluster in a B5 is designed to work for either 4 or 6 cylinders. While the cluster in this particular B5 1,8T could be coded to work in a V6, a V8 wasn't an option. The C5 A6 uses the same basic cluster, and was available as a V8, but as the C5 came later than '96, which this B5 was, fitting one wasn't an option, unless a whole bunch of wiring was on the cards...

...It wasn't.

The existing cluster was pulled apart, and certain key components were replaced, (thank God for Maplins) so now works accurately.

A few pictures of progress so far:

Rusty, porous fuel filter:



New harness in progress. Involves deleting massive auto trans ECU connector, auto trans tranny connectors, as well as configuring the S8 connectors to plug into the B5 chassis. They also needed to be extended from the left side of the scuttle, to the right:





Engine mounts:



Close-up of front engine support:



Front engine support bolted in place: