Congrats on getting the belt on...But, as you describe it, the tensioner is not properly set. The tensioner retainer bolt, (the big one in the middle) has to be loosened to adjust tension. If you don't loosen it, you are racking the rubber coupling inside of the tensioner against itself. Two things will happen. You won't get an accurate setup and you stand a good chance of tearing the rubber coupling loose from the inside of the tensioner housing. With the pointer at the far left, you are a max tension. You cannot set the pointer w/o loosening the tensioner retainer bolt. It can be loosened. It's a big one and has a fair amount of torque applied. When you get it loose, about 2-3 turns after it breaks free. Crank out the adjuster until it is loose. Set the pointer by pushing down on it gently and move it to the far right of the triangle. Then crank down the adjuster until the pointer is at the far left of the triangle. Retighten the retainer to the correct torque and you should have it right. Bummer on the radiator, but better in the driveway than on the highway. These are known for breaking the upper outlet. Make sure your new one has a metal reinforcement sleeve inside the neck.

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Jeff L
1987 300e
1989 300e
1987 BMW 325