I would remove all the rocker arms to be safe. Then I would push the tentioning rail all the way in. If the chain won't reach put a wrench on each cam gear and try backing them up to find the slack. once the chain is together roll the engine through a number of revolutions and set the crank to TDC. See where your cams are and individually remove the bolt and tap the gear off. Turn the cam to the right position and align the gear by jumping the appropriate number of teeth.
By removing the rockers first you avoid any chance of banging the valves.
Bosch Master, ASE Master, L1
33 years MB technician