JB Weld is an excellent product (speaking from experience) I've seen it do a lot of amazing things. However, a couple of your repairs sound a bit iffy. On the carb float support, I have found that JB Weld (like most other thick, putty type epoxies) doesn't work as well when used in a thin film (like glue) as when it is used in a thicker mass. With a bit of luck, you may be OK for a while--JB Weld is unaffected by gasoline and sticks well to aluminum, especially a rough, fractured surface. I've seen it used to repair a cast aluminum motorcycle engine case (dirt bike, after a crash) and it held for years. If clearance is not a problem, and you goop on some extra material around the joint it will be much stronger. I would still try to pick up a replacement float bowl cover---they're not that hard to come by, and I've never seen one break like that. If it does come loose again, it will likely cause the bowl to flood, and gas will run down onto the exhaust manifold---not a good situation. Be alert for a sudden, strong smell of raw fuel, and pull over immediately if you notice it. Keep a fire extinguisher in the car (a good idea in any case)

On the manifold hole, I doubt that JB Weld is sufficiently heat resistant to hold up in that application. As long as the ovesize hole doesn't overlap (or come too close to) the exhaust port itself, it's probably not a major concern.

If you are in a hurry, there is a related product, JB Kwik, that sets in a few minutes, as opposed to several hours. It's not quite as strong, but is adequate for many repairs, and much more convenient.