Need Specs...Desperately

As quickly as you can, get the three books described in this thread. There was enough new for '73 vs. '72 that you will definitely want the '73 factory book; '72 will not get the job done. Haynes and Chilton are garbage.

Tune-up parts and technique suggestions in this thread.

Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.

It looks as if someone has removed the EGR system from your car. That's fine; you'll probably want to leave it gone. Likewise, the vacuum hose should run directly from the carburetor to the distributor without stopping at the OSAC valve which in '73 was mounted to the passenger side of the firewall. Emissions control equipment rundown is here.

As for upgrades: there is no end to the amount of money you can spend. Carefully figure out what you want the car to do (differently, better, etc.) and your time and money budget. Make a sensible list of priorities. Safety should be near top of mind. I'd put in proper seat belts. Involved but not impossible in a car like yours. If you're not keeping the original seats, the easiest and best way forward is a set of Chrysler Sebring convertible seats with their built-in 3-point belts. Putting these in requires some electrical work; see here. If keeping the original seats, I recommend these up-to-date, reputable-brand ECE-approved 3-point seat belts (linked vendor is reliable; I've been buying there for a decade; their eBay store is here). More info on belt selection and mounting is at the maker's website. If I needed mounting hardware, I'd buy it again from Wesco (and their page has good illustrated write-ups on how to install belts in pillarless cars like yours here ), but the Securon belts are a much better-designed product than anything Wesco carries.

There are also bench- and bucket-seat 3-point belt setups available from XV Motorsports, but I am hesitant to recommend or use them until I have more than a one-line say-so from the company that their belts actually meet the relevant safety standards; discussion on that matter is here.

Unless the brakes are newly (or almost newly) redone, I'd put them near the top of the list for upgrade. 9" drums at all four corners were adequate for the original lightweight '60-'62 Valiant and Lancer, really not adequate in today's traffic on the heavier '63-up cars. LOTS of options for disc brake upgrades; ask for advice as a separate thread.

Good shocks (at least Edelbrock IAS, preferably Bilstein) and good tires (not from China) go without saying.

Of course, being me, I would put in better headlamps (Cibie H4s if you want good but costly, GE Night Hawk H6024NH if you want decent and cheap but not very long lived) fed by relays, and would also upgrade the car's other exterior lights (see here).

Then after making the car much less inadequate to drive in today's traffic, I'd then turn my attention to more extensive performance upgrades. Perhaps a 2bbl intake and carb either with factory parts or with a home-modified manifold; see the parallel 2bbl setup article -- photo documentation of a very well done such conversion is here. And a nicer exhaust system with 2¼" headpipe. Stock manifold is fine, Dutra Duals are better.

For more advanced engine upgrades: You'll hear the name "Clifford" come up, but be very careful before you decide to spend any money with Clifford. They have a long and ugly reputation for being a bunch of clowns; see for example here, here, here, here, and here. Much of what they sell is inaccurately described, and a lot of it is not even slightly cost-effective. The good news is that you don't need to go to Clifford to get hot rod parts for slant-6s; there are lots of other, better options. See for example Dutra Duals and header options discussed in this thread and this one, Erson custom cams, HEI ignition upgrade, Mike Jeffreys windage trays, Hurricane intakes, other exotic intakes. Hi-perf engine buildup here, high-perf parts and build info here.