If you are welding a crack on the chasis, there is a procedure for it, and as a certified welder, I have had to this a few times now.
First is dertermine where the crack starts and stops, then drill a hole at both locations, this is required because when you start welding the crack could continue cracking and the holes stop this.
Before welding preheat the area, this is required because most chasis's are made from a high grade steel and the heat generated could cause a crack to form next to the weld, 80 deg up to 150 is recommended. I use the touch method where you heat it and when it is too hot to touch that is enough. A heat gun like what you use to remove paint is good for this.
A chasis is usually coated in antirust and road grime so clean it first, with a grinder then thinners but if you preheat that also gets most of the crap off.
If you are not aware of the fact any modifications or repairs to a chasis are required to have an engineers cert, but if you are good with a grinder and can hide it with paint you can get way with it, just dont leave it in an obivous state that screams 'look I've been worked on' plus the welds that you do MUST be of good quality otherwise you are just creating a weak point that will fail eventually because of the flex that a chasis continually goes through.
I hope I havent you discouraged too much but they are very important part of the vehicle and must be treated with care and attention.