Since the inner rubber seal slides over the end of the column for installation, and since it won't slide over the shifter arm at the base of the column, you basically have to disassemble the steering column to replace it. Therefore, it would be wise to replace the seal with the proper one while everything's apart, since it would be a shame to disassemble the steering column to slide a home-made seal on, button everything back up and reinstall, only to find that it wasn't working and you had to re-disassemble everything to replace the seal anyway.
The rubber seal in the DC catalog specifies just for manual steering...they don't show one for PS:
Here's a photo of the MS seal:
The seal that was on the PS plate above was 99% gone, so I couldn't compare it to the MS version, and now that I know the plates are different, I guess at this point I'm not willing to say that the MS seal will work with the PS plate.
The '78-'79 tilt columns do not use the outer mounting bracket, only the inner one, which doubles as the seal retainer and mounting bracket. Since I believe the tilt columns are a different diameter than the earlier non-tilt versions, they'll take a completely different seal. I'll take a look my tilt column and report back.
No, you don't have to enlarge the hole in the firewall when going from MS to PS. It's a bolt-on procedure providing you have the correct brackets. There was not a MS and/or PS firewall, only one firewall was used for all applications.
Robert: Yes, there are two brackets. Well, actually, the one on the outer firewall is the only mounting bracket. That's the one which holds the lower end of the steering column with a clamp. The inner seal retaining plate is just there to hold the rubber seal....the rubber is the only thing touching the column, so it can't really be considered a bracket. This is the outer bracket:
The one on the left is the manual steering bracket, the one on the right is for power steering.