There are a couple of ways to go. One is as Steve B suggested - removing the rockers so that the valves don't move as you turn the crank/cams. The is a K-D spring compressor that you can get at a good auto parts store that works very well.
The other way to go is to determine which side you are hitting on, and move one link back on that side. You would remove the sprocket, move it back - CCW- in the chain and turn the cam back enough to line up the sprocket and put it back on.
I think that this will work because what is happening is that the valves are opening too soon relative to the crank and turning back a notch will close the one that is hitting enough for you to rotate the crank to TDC.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe