hi john
both cars respond very well to LPG conversions and there are members on here with both - there is hardly a case of engine problems.
the deciding factor is which car to buy:
the mark 4 lexus LS400 (97-00) is the understood to be the best in terms of reliability - The Mark 3 (95-97) is not far behind engine and comfort wise but doesnt have the VVTi engine, touchscreen controls, xenon headlights and VSC.
The 430 has more toys such as :
keyless entry and start
heated and cooled seats
rear blind
reversing camera - 04 onwards
soft close doors
rain sensor
rear reclining seats and vibrating
it does have some weakness over the 400
exhaust is not stainless like the 400
air suspension - most common failure is seized ride height sensors prevents suspension from adjusting
steering motors prone to failing
early 430 are at risk of gearbox failure due to the design of the rad where transmission fluid leaks into radiator
at least one parking sensor fails causing constant beep therfore have to switch sensors off
if you decide to go for a 430 try to go for a newest one possible with full history - cambelt must be changed 10 years or 100k miles
hope this helps