Can a new fan shroud be installed (on a '78 L48) without having to
remove the radiator?

I think so.  On my '81 in can be and our cars should be real similar.  Once you get the fan and some accessories out of the way, you just have two bolts at the top corners holding it in.  The bottom is held in place by two tabs on the bottom of the shroud that insert into two slots in the radiator support.  It will still be tight getting it in and out, especially around the right side upper a-arm, but if you get it in just the right position, and maybe flex it a bit, it'll come out.

Just a thought... if it were me and I was that far into it, if it has the original radiator (you didn't say), I think I would have to consider replacing the radiator too, so I didn't have to do the work again any time soon.  I put a Dewitt's direct-fit aluminum unit in and my coolant temp went from 200 - 210 down to 180 and oil temp from 250 to around 180.  I couldn't beleve the difference.

I've got an '80 and once the fan and clutch are out of the way, it comes out relatively easy...except for the upper rad hose.  Not bad, just like Chesh said, you have to flex it a little.

I took my original rad in and they tried to flush it, ended up rodding and coring.  Temp is down but not as good as what a Dewitts sounds like it will do.

Either way, it'll be worth it to have it looked at or replaced.  Oh yeah, the xmission lines on the passenger side are much easier to get to with a wrench if you have the lower hose out of the way and they have to come out to pull the rad, not the shroud.

I just fabricated a piece of 20 ga steel for the bottom of my shroud, it had a gaping 7 X 12 inch hole in it.  Sealant and rivets and it is working fine.  Let's me spend $$$ on other stuff like the distributor parts.

Dennis