I've been on Lexus forums for a lot of years and I don't remember anyone using Mustang wheels on a Lexus LS - not that it can't be done.
I don't know how many versions of these Mustang 17x9" wheels there are but these I found have a 24mm offset: http://www.oewheelsllc.com/Mustang-Cobra-Deep-Dish-Wheel-Gunmetal_4?gclid=CjwKEAiAn7HEBRDHwNqitoWqsQcSJAADWmI29Eyk1GeTXbL-Fs3zB5-08JBIP6hbtbnEqyUT6oUBZRoCfuLw_wcB
Wheels and Caps, where I've bought refurbished OEM wheels, show 1995 LS400 wheels as having a 45mm offset: http://www.wheelsandcaps.com/p-20690-aluminum-alloy-wheel-rim-16x7-74140.aspx
That's a big offset difference.
The "normal" tire size used on the 2001 LS430 with the 17" wheel option is 225/55-17 which keeps the speedometer accurate and reading the same as with the standard 16" wheels and 225/60-16 tires which was the tire size used on the 1993-2000 LS400. According to 1010 Tires, the widest rim for a 225/55-17 tire is eight inches: http://www.1010tires.com/Tools/Tire-Size-Calculator/225-60R16/225-55R17
A local wheel or tire shop might be able to help you with this. If you have found some used Mustang wheels locally, I assume you could test fit them.