I made these little filler pieces to put inside the pipe joints. They do a couple different jobs. First they help hold and align the different sections, and two they stop your welds from blowing through. Especially handy with mig. If you're tig welding them you probably wouldn't even need any filler wire.
Front pipe starting to come together
First piece on the rear pipe. Note shitty tacks on pipe flange. It's a bitch trying to weld upside down!