Hello.

I am sorry to ask this basic question but I searched and searched again but could not find a clear answer:

What is the offset of a stock 1991 LS400 15x6.5 wheel?

I'm far away from my car and cannot unfortunately measure anything.

+45 iirc

Really?
A JDM 91 Celsior's 15 inches wheel: +50mm. See below. The exported models should be the same.
http://spectank.jp/whl/001110827.html

I also read 45 for '92.

Thank you, gentlemen.

Is the offset number by any chance stamped/cast on the wheel as it usually is on aftermarket wheels? I'm still away but could check this once I get home.

Yes. Just since I looked at the factory 16's, the offset is +45.