I'm working on my first gasser too. I think the connection on the bottom of the Walbro carb is supposed to be conected to your crankcase line. The pulsing effect of the crank and piston is what works the pump in the carb. There's a little diaphram in the carb and the pulsing line from the crankcase works the diaphram to make it pump.

You will not need tank pressure with the Walbro carb. So, you only need a clunk and a vent. Some guys use a third line to fill the tank.

I'm no gas expert, there are plenty of them lurking on these boards. I'm sure you'll get plenty of help from more experienced guys here.