I just found some nice 18" wheels with 245/45R18 tires.
I just looked up that the stock sizes are 235/40R18 and 245/35R19.
Would this still fit?
I have some pretty hefty HIFI stuff in the trunk that weighs down the rear, but also installed lifting springs to combat that.
If it wouldn't fit, could I get it to fit with some offsets?

 

I am currently using 255/40/18 on stock wheels without any spacers. However the only reason I'm using them is because I found an excellent deal, 4 tires for $400. They are a high end Pirelli. My S80 started life with the optional 245/40/18. If you want higher load carrying capacity then buy tires with the load rating above 100 or a number as high as possible. They won't provide as smooth a ride. If you want 35 series they might not be available with high load ratings. 35 series are not usually designed for loads but for traction and for most people to look cool. I think wide tires are cool but at 72 years old it's just not high on the importance list. So be careful in your choice of tires. The 245/40/18 is easily wide enough for handling purposes and you can get them in higher load ratings. Have fun but don't blow the windows out with all that sound power.