The SC300 and SC400 (but NOT the SC430) are the same body and many of the parts are identical. I do not know for certain, but there is a very good chance that a windshield that fits an SC300 will fit an SC400. They might even be the same part.
All I have heard is to replace it.
Yes
How do you install a oil pan gasket on 1995 Lexus sc400
I use the Denso plugs in my SC400
Yes they will
You can reset a maintenance light on a 92 Lexus SC400 by disconnecting the battery. Leave it disconnected for 10 to 15 minutes.
Yes
5w30
250hp @ the crank
$41,139
Yes you can.