I want to buy some tires for my M56S and I want to get 245 45 20 up front and 275 40 or 45 20 in the back. I want the back tires to sit out a little, maybe 1-2inches past the back wheel well without rubbing. What's size should I get for the back?

I suggest you use the calculator below. I believe your car came with 18 X 7.5 wheels with a "+45" offset. Assuming you have upgraded tkol 20" wheels, you will need to know the width and offset of those 20" wheels. Use the calculator and enter your wheel size, width, offset, and tire size in the left column. Then you use the right column to enter whatever new tires you are considering and the calculator will show you how the new tires will affect wheel well, suspension, clearance, speedometer accuracy,etc.

http://www.rimsntires.com/specspro.jsp

K. Thanks

A 1" to 2" bulge is way too to much
Unless you plan on making your suspension almost rigid I don't think it'll work.

You'll need to do some serious mods to that suspension that won't make driving the car comfortable in my opinion.

I would take it to a place that does some racing and will inform you that it will not work well if there is even an option for you to do.
I put on 275 35 20 on the factory wheels and I also have 1" spacers on the vehicle. I don't get any rubbing and it looks good in my opinion.
Good luck
Let us know what you end up doing
Andy