That airplane was probably Oxy Acteylene welded, Most steel tube aircraft that were built before 1986 were. Boeing still Oxy hydrogen welded Aluminum until the mid 1990's. One manufacturer mig welded and then had to heat treat the entire airframe in a huge oven. Think of 6'x6'x22' oven that got the tubing to a dull red. When those airplanes crashed, they usually broke in the weldment.
Tig used to be required to be pre and post heated to a dull red with a torch and then left to air cool to normalize 4130. Some will argue this topic but many airplanes are built from 3/4"x.035 wall 4130 normalized tubing. Engine mounts are .049 or .058 wall. They hold a 500 lbs 500+ cubic inch engine that puts out 750 lbft of torque at 2700-3000 RPM. They only get around 235-350 horsepower. All of this on around 8.0-1 compression.
I actually read how NHRA got to require tig, the called a welder up and he suggested TIG. They make great welds. Read the attached article. Tim

http://www.netwelding.com/Welding%204130.htm