[Wynand]

If I may chine in,

Being a boilermaker for 35 years I had my share of welding and paid my dues.
The first thing to do with your chassis is to keep it form twisting/warping whilst welding.
Get the chassis clamped down good - in my case I built mine on a 1.25m x 2.5m steel table with 10mm plate top. The frame was tack welded to that before welding.
If you do not have the luxury of a steel plate to attach the frame to, clamp to wooden table but be sure the wood is fasten to solid frame.
Start welding frame as she sits on table/jig. It is important to stagger your welds - IOW, do only a 25mm run on joint, move to other part of car, do the same etc. This is to prevent excessive heat buildup on a joint/steel that may result in warping when the steel cools and retracts - it is cumbersome but the results well worth it. This is the most important stage of the welding issue. (test: take a half meter off cut tube, do a few welds on one side only and see how it pulls into a banana shape)
DO NOT WELD ONE SIDE ON THE CHASSIS ONLY - this will result in a car good for scrap - stagger welds as mention all over chassis.
About 80% plus welding can be completed whilst the frame is secured and when it is taken off, it should be straight, turn over and weld bottom welds.

A MIG welding machine makes this very easy since weld can be done vertical down without any loss off strength but do not do this with arc welding. There will be a lack of penetration and chances good to trap flux in welds...
If using an normal arc welder, err the amps a bit on the high side as this simplify vertical up welds when using broken arc technique which is the way to go with the thin wall steel tube used on chassis.

Hope this helps.