Build a long-wheelbase low racer recumbent bicycle

Step 7 Weld up the bottom bracket and front end

This is one of two tricky welds. A jig would help tremendously here, but for a one-off bike, I personally wouldn't make one (I want to make bikes, not jigs). With careful clamping, you should be able to clamp a straight piece of metal (like the leftover 1 1/2" square tube or the 1" conduit) or a straight 2x4 across the down tube, the top tube and the new main tube to keep things straight while welding.

The proper angle is determined by the bottom bracket measurement you made at the beginning before you cut the bike apart. make sure the wheels are on the frame and inflated, make a straight line between them and adjust the angle so the bottom bracket will be the proper height from the ground. If you have feet larger than size 10, you might want to add 1-2" or your heels may scuff the ground when you pedal. You could measure your shoes and make sure you have a good 1" clearance. This is very dependent on the donor bike and can be adjusted later (but do it now - it's easier and less work).

Now, weld it up. Spot weld on one side and sight down the frame and make positive that everything is straight and that there are no twists. I was able to keep things straight easily, but I think I just got lucky. If it's off, you should be able to bend it and tweak it a little. If not, grind the weld away, re-adjust, and try again. With a long bike like this, it's pretty easy to eyeball when things are off. Once everything looks good, carefully finish your welds. Don't weld it too hot (and the bottom bracket should be removed from the shell already). Be careful not to get slag in the shell threads.