Speaking as a pretty second-rate welder, I found it really useful to build the bulk of the frame using just one spot of weld at each joint. I then used a series of Spanish windlasses, sash cramps, etc, to pull it all square before going round and building up the welds. Diagonal bracing followed. I cut all the box section as accurately as I could using an angle grinder in a cheap stand but it wasn't perfect. Your cutoff saw might do better. And if it goes pear-shaped (literally!) then there's always the angle grinder to cut the weld to start again - another reason not to weld too much before you're happy with the shape. The great thing about welding is that for something like this, it is very forgiving. I cut one piece on the wrong side of the line, but built back up with weld and ground back to dimension...