The location of the #1 cylinder does not really matter for installing the rocker arm assembly.

You will want the #1 cylinder at TDC in order to set the valve lash. First, set them cold. Then, warm up the engine as described in the procedure. Then, check the settings hot.

Here is a valve lash adjustment procedure in the Stovebolt.com Tech Tips.

Here is another write-up for adjusting valve lash.