This discovery has me really worried--so what better medicine to take for
this headache than advice from the shark doctors?.......and that would be
you guys.

Really got under my '78 for the first time yesterday, and I found
something that's broken. I know it's serious, but I just don't know how
bad it is.

On the driver's side front, above the lower A-arm, in the frame structure
through which the shock and spring are mounted, there's a huge crack in
the horizontal metal plate. It runs for several inches outward from the
opening for the shock/spring, and there's a good 1/4" or so of separation
in the worst bit.

This isn't a bolt-on piece; it's an integral part of the frame, although
there's clearly a seam around it, so it's appears to be a welded-in plate.

My questions to you all are:

How serious a problem is this? Meaning, is this something that can be
'lived with' indefinitely, as it appears to have been for some time, or is
this something that effectively makes the car unsafe to operate? (We're
not driving the car right now--it's very much in 'resto' mode, and will be
for some time to come. But eventually...)

Is this replaceable/repairable--maybe by expert welding? Or am I looking
at a frame that is essentially shot?

I'm really worried about it. Our goal is to bring this diamond-in-the-
rough back to better-than-new if we can. But if a new frame is needed,
that's going to put an end to the dream, at least for the forseeable future.

Almost any frame can be repaired.  Almost.  It's difficult to answer your question without some visual aid, but sound very repairable.  Can you post a picture or send me one in an e-mail?    bunky44221@yahoo.com    Then I can give you a much better answer. kstyer38867.4803356482

It is very common for the lower a-arm support brackets to break loose from the frame. Seen it plenty of times. The welds from the factory are not all that well done, and that contributes to the problem. Hard driving, and large tires will contribute, also. It can be fixed, tho.
Any competent body shop could do it, and some quality alignments shops can, too. I would suggest an alignment shop, so they can get it all back in position correctly, before welding the thing back up. Check the other side very carefully, as it may have cracks too.
Look around the frame on the driver's side, where the power steering cylinder connects. There is a bracket there that the cyl. goes thru, and then that bracket is bolted to the frame. The frame will crack there a lot, but it too can be repaired.

Sounds like the spring support housing is cracked.  Should be an easy fix for a competant welder. I agree with Joel to get the whole front end checked.  Sounds like some serious bottoming of the front suspension could do this, so I'd be checking how straight the chassis is as  well.

Thanks all for your replies. I will try to get a picture of this injury ASAP.
The lower A-arm and its attachment to the frame don't appear damaged
in this case. Rather, it's the plate, almost horizontal to the ground, in
between the upper and lower A-arms, in which there's a large opening
for the spring and shock to pass through. It's essentially the outward
extension of the engine cradle, on the driver's side.

The damage is such that the edges of the crack are jagged and uneven;
and peeled away in both directions, up and down--so it must have been
due to some serious impact (?).

Strange thing is, to the naked/amateur eye everything else in this area on
the car appears basically OK. The front end shows no signs of serious
collision damage or major repair (I'm certainly no expert, but I've
examined it pretty closely now and don't see anything alarming or
suspicious--but I also know that even small variances in alignment/frame
straightness can throw off the suspension geometry; so I will have the
whole setup checked by a competent shop when I can).

I'm reassured to think that it can be repaired. I guess it's just as well that
the car's currently not in driveable condition anyway. I just hate to have
something torn up on it that I can't attempt to repair myself--just goes to
show how attached I've gotten to this in-such-sad-shape shark already...

but then again, who'd understand that feeling more than the crew of you
on this forum?