Weber install questions

I just installed a weber carb on my '79 D50 2.0, 4 speed. I have two fuel lines that run into my old mikuni carb and one return line running out. The new weber only has one fuel inlet. Should I just cap the return line and second inlet line to the carb? I read a couple posts about the conversion but nobody really touched on this.

I am swapping to a Redline Weber K675M, and Pacesetter headers. Stock motor with 5-speed. I have a few questions that I cannot seem to get any answers to, so hoping someone who has done this before, can help a fellow B2200 guy out; thanks in advance.

1) I watched a You Tube video for swapping the carb...pretty straightforward. I already made the plate to cover the EGR valve (yes, am strip all emissions crap from the motor). My question is two-fold: There is a vac line coming up along the two fuel lines from under the cab; where will this line be attached? And there is a Green threaded plastic plug (with wires), on the front left side of the intake; what is this, and what do you plug it with?

When installing the header, do you only use the supplied gasket, or should you use the original metal gasket...or a combination of both?

3) What size are the header bolts, to replace the stock exhaust bolts (can't find the size anywhere...Mr. Gasket offers lots of header bolts, but need the metric size)

4) What size bolt goes into the header collector pipe, to fill the hole where the O2 sensor would go (if using emissions)?

5) What is the best muffler to let this setup breathe good, but not sound like a motorcycle?