Pretty much right on about the plumbing comparison... There are a few simple rules that'll also help when doing slip fits... Some require welding or brazing, no big deal there, just weld or braze what's needed and you're good, for slip fits that are close to the eangie or where there's still a good amount of heat, the smaller diameter pipe should always be closest to the engine or the heat more importantly, then the next section slips over the header section, the header shouldn't slip over the next section because a good slip plus the heat expansion will make a leak free slip fit, loose enough to pull apart when the engine cools down, but tight and leak free once warmed up.
For making a small bend or to change the angle of a section of pipe where the length needs to remain the same, you can pie slice half way thru the section and then straight cut the rest of the way thru and then use the cutout piece on the opposite side, this will ensure there is no change in total length when making a bend, this is no big deal if only one bend is needed after the pipe is assembled, but if there's 5 or 6 bends needed, each 1" pie slice effectively makes the pipe's total lenght 1/2" shorter, but by cutting half way thru and putting the pie slice back on the other side, the pipe basically pivots on the centerline and doesn't lose any length.
I also prefer brazing the pipe where there are a lot of pre cut angles assembled to amke a band since each weld bead can protrude slightly into the pipe but when brazed, the brass typically fills the joint but won't sag into the pipe... unless you're a highly skilled thin metal welder and can keep the weld from protruding into the pipe where you can't see it. A braze joint is also typically smoother so it won't disturb the flow as much.
I also prefer to TIG weld the piping when brazing asn't practical because you can tig weld the pieces together without using filler wire whenever you weld up a slip fit where 2 pipes overlap by a small amount which will also ensure that no metal sags into the exhaust path restricting the flow.
