I followed the instructions and installed the DS before I dropped the motor in. The biggest issue I had was I didn’t have the motor tipped back so it was a struggle to get the motor back far enough for the oil pan to clear the k-frame so I could drop the motor into the mounts. The header was hitting the firewall before I could drop it further and having the motor tipped back would have been huge. Next time I will have a way to tip it.
The PS header slid in from the front without issues. Partly because the alternator wasn’t installed. Some of the bolts weren’t easy, but not impossible.
I will add that I don’t see a way to install or remove the DS header without at least tilting the motor and maybe even removing or shifting the steering box.