One thing many DIYer make as a mistake is to put the metering diaphragm next to the carb body when the order is gasket and then the diaphragm. As the metering lever height it level with metering chamber floor.
And you're correct that you got to use a sharpen punch to get the welch plugs out. But I seldom ever need to remove them as I use an ultra sonic cleaner to clean these carburetors; although, I do like to remove the mixture screw(s) when I am cleaning the carbs which allows the USC access to the back side of many of the welch plugs.
And here is the service manual link for the Walbro SDC.
SDC Service Manual
