Hi Nelson. Congrats on buying a great car, I'm pleased to hear that it's as trouble free as it should be and boy - YES - does it turn heads
The AM DB7 uses XJS underpinnings which pre-date the Celebration. I believe the engine uses the 3.6 block bored out with a special AM head plus supercharger. Suspension and floorpan are I believe all XJS with AM mods.
There were many changes over the years to the XJS, in fact it's a study in constant development. I expect you're more interested in the overall picture than trawling through mod. sheets. There are several good books which cover this including the Haynes "You and Your XJS" and other books, most of which can be found on e-Bay. All are worth reading as each seems to pick out details or have pictures which are missed by the others.
Al - I hope that I am not one of the guilty parties posting complete rubbish about the facelift cars, but if I am then I ask your forgiveness
Tony