Do you have the bolt to hold the cylinders at TDC? This is the one that screws in under the water pump? It really is necessary. You can make one your self. Also do you have the Workshop Manual downloaded from this site?
You can do both at once. You could just replace the rockers without the shims, or just turn it. A screwdriver through the spark plug hole is good enough for TDC. This is where the bolt really helps.
I would advise you to remove the radiator for this job and don't forget to re torque the head. You need a special tool for this also as there is a hard to get at nut. These tools are not expensive as there is not much to them so they may be easier to buy than to make. Good luck.