I'm not quite on board with yanking the engine, but here are my thoughts and experiences shared;
Scott will make it perfect. If the body is warped, he can do something with it. New ones are available. Gets them tuned right on a mule engine;
http://www.harmsauto.com/
I just bought the 1901 Street Demon for a 318. 625CFM. Holley/ BG/ Demon makes a 750 Street Demon (1904)
If you go this route and you have metal line from your carb, Holley does not supply a reverse flare in a 5/16" with the kit. If your car has 3/8 steel line to the carb, you are fine, they come with that. Holley is the only one that sells the 5/16" reverse flare fitting, if that's what you've got, currently, to make it a clean install.
The carb will work with your air cleaner, no problem. I'd likely get Scott to redo the Carter TQ. The problems will go away. He just about has a monopoly on old TQs and does beautiful work. You can't go wrong, either way with that car, though. The Street Demon is a superior design, in my opinion, after going through mine for jetting and seeing the two, side by side, but you've got the next best thing that is dead nuts original, so that's why if I were in your shoes, I'd personally keep the TQ and send it to Scott.
Pulled a leaky P/S box from a '73 Rallye Charger U code 440 resto about two years ago that was finished. If memory serves right, you go out from beneath, but I don't think the torsion bar comes out. Just the center link on that side and maaaaybe the exhaust manifold (which you're doing anyway). I know we had no issues leaving the engine and torsion bar in, though. Just fix the seal. Don't core it to some place. They will send you something in return that looks like it was dipped in tar and that crap doesn't come off easy.
Rent/ borrow a coolant pressure leakdown kit and find it. Do this before you do anything else. You have to determine that the block is sound. If not, you need to get it stitched at the very least, at which point, I'd also pull the intake, leave the heads and examine the inner jacket walls. They are prone to cracking, but usually from freezing. No antifreeze in your oil tells me you just have an outer leak. Finding it is as easy as using a pressure kit at about 13-15lbs.
Just replace the $2 studs. Vise-Grips and thread lock. Check the surface of the manfold and head and clean them up really well with a plastic gasket surface cleaning wheel, after you pull the studs and do both sides. Get sufficient head tubes, new gaskets on both sides and be done with it.
I really don't think you need to go this far. I think a large portion of your frustration with this car has directly to do with the small things that are wrong with it. The carburetor and a good tune (ignition included) will likely put a smile on your face, regarding how it performs.
I know the easy answer is a fix-all and to pull the engine, but believe me, on a car like that, you don't need it and it's way more work than what you set out to do with the engine in the car.
Consider this; in some likelihood, you may rebuild the engine and have to chase other leaks out of it, after the fact, anyway. I personally think that pulling the engine is time and money spent better doing other things.
I think you should fix the leaks, track the belt squealing and correct it, rebuild the carb and see how you like it before going cowboy on it. This is coming from someone who does it all the time and had to rebuild an engine, basically to exactly what you are talking about, Demon carb, new cam/ upper end and all and I only wish I could have done it in the car. It would have saved me a lot of time and unforseen problems.
Start small and move from there. If you have to pull it after trial and error, at least you'll have the carb tuned to go back on.
