So make sure the chain is tight over the cam sprockets and under the idler sprocket when you line it up. Then put the tensioner in, turn it over by hand a couple of turns and check the marks. Then pull the coil wire and turn it over on the starter, and make one final check.
Can I ask why you are not replacing at least the chain and possibly the tensioner as well? It's inexpensive insurance and fairly easy to do at this point.
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