Typically there are four bolts in the corners that need to be taken out to remove the float bowl.
As for your power valve size should be half your max idle vacuum - so if you are pulling 13" then you can get away with a 6.5 power valve.
I'd check your jet sizes which seem to be maybe a bit high as well.
Here is some more reading:
How it works:
The power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the main power circuit. The valve itself is a small rubber diaphragm with a small coil spring. When opened, it allows fuel to flow through a calibrated opening in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.
The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.