As far as the original '99-06 S80 and the '05-07 500 go, there is very little shared between these Volvos and Fords.Want proof? Pop the hood of any S80 and 500 and take a look at the engine bay--not just the engines--then put both models on a lift and take a peek at their respective undersides noting the differences between each model's suspension as well.
They're not copies of each other. Ford took exquisite liberties to produce a decontented budget version of the S80 to satisfy Ford's down-market needs. AFAIK, what Ford really wanted was to build a car that rental fleets everywhere would buy in huge numbers much as they did the defunct Taurus. Furthermore, Ford *had* to take such liberties with the 500 to bring its price point down to what consumers would expect from Ford.
Ford's idea to spawn the 500 from the S80 would've actually made way more sense if it had been intended from the start to be a higher-end Lincoln-only model--like a successor to the long dead Continental or--even better--to the Town Car which IIRC is based on a platform that's nearly 30 years old.
Figure that the typical Ford family sedan buyer really isn't looking for a car that's priced like a fully loaded S80--s/he simply wants a car that's priced more like a Camry, Accord or any other Consumer Reports-approved appliance-on-wheels.
Speaking of which, so lackluster were the sales of the current 500 that Ford will be renaming it *gasp* Taurus for 2008 along with a tepid restyle and a more powerful V6 to hopefully make up for the woefully flagging market performance.