My 73' 450 was also garaged for some time (5 yrs) before I bought it. I contacted a flyer buddy of mine and got some 100 no-lead from him, running a full tank of that through cleaned the inside up real well.
Next thing you will have to look out for if your luck is like mine. The fuel hoses on top of the engine, along with the injector boots may have cracks in them.
Check these over well, you would not want your SL to burst into flames while driving down the road. Mine struck a leak on the main injector line and spewed gasoline all over the top of the engine under pressure. Good thing I was not far from the garage.

Good Luck

Housesurgeon@earthlink.net