To be honest I find tube notchers more faffing about than flap discs on the grinder. Even those fancy elliptical machines such as Balaigh, etc, sell aren't as fast as freehand, great if you're doing 100 notches the same, waste of time for a one-off spaceframe with all sorts of angles.
you can get a simple tool called "pipemaster" its an over-fit tube full of sliding needles. fit this up then copy the shape required onto the end of your tube and go easy cutting out with a hand held grinder .
Tube notchers? I shouldn't bother with personally for a first go one off job on a joint with so much going on like this, you as likely to confuse yourselves
As other have said fit -up is critical, if your not having a laugh you should be
I fancy a trip out to Cairo, if your UNI is happy to foot the bill I'll be more than willing to pop over for a week or two to assist you with your frame welding job.
Is there a Hilton Hotel nearby?