Hi SELfour50, that is really excellent news and good luck with your purchase. The car is probably about 200 kgs. lighter than a standard 6.9, and with the lower (higher) diff. ratio, it is bound to have some decent get up and go.
I'd imagine you'd be well and truly into 6.3 territory, if not better.
Is there a chance of some under bonnet and interior pics ? How did the previous owner handle the dry sump arrangement ? I take it that it is still in place. There is a good friend of SEL69L and myself here in Sydney who has also fitted a 450 diff. to his 6.9, but everything else is still standard. However, he does swear by the improved acceleration of his car, and maintains that the high diff ratio is pretty useless around town and the suburbs. Anyway, would really be interested to see more of your car, whether by way of pics or even in real life. Test Drive mandatory !
Regards Styria