I don't think this is a "told you so" moment. Sure, some of those issues would have shown up with a thorough inspection. And any time a car has had a bunch of work done a thorough inspection is necessary because you don't know what corners "the other guy" cut. But some of the issues you bring up are probably a result of driving the car more. I think a lot of people, myself included, tend to forget that most people don't drive these cars all that much. Especially if we're about to sell them. Then the new buyer goes out and drives the car all over the place because it's new and fun, and issues that have been lurking pop up. Yes, sometimes the previous owner is a shady bastard. But sometimes they may not have known that a bunch of crap was about to go wrong because they'd had it parked for 6 months. Same stuff would have happened to them if they dragged it out and put a few hundred miles on it all at once.
Anyway, onto some of your problems. How did you figure out the car needs a timing chain? What was it doing that made you suspect that? You last said the car ran great across the desert...
As for the ignition, toss the MSD box and just buy a "plug and play" electronic distributor. There are several out there, I've run a couple of the Pertronix Flamethrower billet distributors. They're nice, they're easy to install, and they get rid of all the big ugly boxes and ballast resistors. Sure, there are more tunable options, but based on the info you've given so far you don't have some wild engine build so more than likely a plug and play distributor will be more than up to the task.
On that note, a 600 cfm carb should be just fine for a mild 360. Even a moderately built 360 that's driven primarily on the street will do fine with a 600cfm carb. Yeah, you might lose a little HP at 6k, but again, for a street car you're not going to miss it. A 750 will work too, but I would caution against changing too many things at the same time. Figure out your timing and ignition first, then move on to the carb if you feel like you still need to. Throwing a whole crap ton of parts at the thing all at once is a great way to make a tuning nightmare for yourself.
Where is the gas tank leaking from? The sending unit gaskets and lock rings on these tanks are pretty finicky, if the last guy installed a new sending unit that may be the culprit. Also check the fuel and vent lines, fuel hose isn't what it used to be and the cheap stuff can harden and cracks in just a few years time. I would suspect all of those things before the tank itself.
For headers, I like the Doug's D453's. You can get them at AutoZone and use their 20% internet discount that they frequently run, makes them a lot more affordable. The TTI shorties don't fit any better than the Doug's, I fit a set to my car and went back to Doug's. The TTI shorties interfere in
different places than the Dougs, but not really
fewer places. There are advantages and disadvantages to each, some of it depends on how your car is set up. I did a write up on that install in my build thread, this should link that post
My "new" '74 Duster- or why I need a project like a hole in the head. Regardless, I wouldn't waste my money on anything other than a set of Doug's D453's or the TTI long or short tubes. Other than those 3 everything else is pretty much garbage for a small block in these cars. It may just be my imagination, but it seems to me the Doug's fit the later spool mount K frame cars a little better than the TTI long tubes. That's not scientific or anything, just seems like reading threads here that the Doug's fit the 73+ cars a little better.
