James,
No welding please. The longeron is no place for that. You can splice in a piece if you must (per AC43.13 techniques), but the best bet is to just go to your local metal distributor and buy 2 brand new pieces that are long enough.
Now, the original HRII longerons were 3/4x3/4", and this size is well proven. However, some stress analysis has shown that the 3/4" size is a bit on the light side. So, upsize it to 1x1" angle if you like.
This will make it MUCH easier to maintain edge distances, particularly when installing the tail. It also reinforces the area just aft of the firewall where the potential weakness shows in cad analysis.
It's possible to mill down the 1x1 to 3/4x3/4 in areas between the tail and forward fuselage if you want to shave weight.
The F4 that I'm building for my personal flying will use 1x1.
YMMV!