quick subframe connectors question

hey guys
i am getting ready to build some subframe connectors for the duster

i think i got a pretty good plan (looked at the BBdart page, and i got a copy of the plans one of our members made, but i got just one (or two) quick questions

the big one is the fuel line
i will be using 2x3 box steel, and it looks like if i tuck it over the rear frame rails i will come in pretty close to where the fuel line runs throuh the front sub frame

how close can i safely get to the fuel line while welding?

or is there a good way to shield the fuel line from the heat and sparks while welding?

and secondly, the plans i have call for "plug welds"
is that just for added strength?
it is simply a hole i weld in, correct?