Unfortunately, this isn't good. The diaphragm shaft should have come out cleanly, but what you got is the center of a rubber disc that is the vent valve. Here's a diagram of the bowl vent system. The circled part is the rubber valve:

Bowl vent valve.jpg

The center of the rubber valve tore out because it was brittle or degraded with age. To replace it now requires removing the carb and it's top portion. But if you're going that far into it, and given the amount of internal crud seen in the pictures, it may be wisest to replace the carb. The Mikuni is very difficult to rebuild successfully, and rebuild kits don't include the rubber valve, which I haven't seen available in decades.

Most site members recommend going to a Weber which is about $300 new. A rebuilt Mikuni is another $150, at least. If those are out of the question, I have a good used Mikuni that might work, but I need to know if your truck has an automatic or manual trans.