I have only built a bike jig, which didn't require the same degree of resistance to twisting that a car jig would require. But I beams or the more commonly used wide flange H beams are not good at resisting torque (twisting). Hollow structural square, rectangular, or even round would be a better starting point. However, you have a start, so you may be able to box in the beam to make it much better in resisting twisting, if the welding doesn't warp it. Whatever you use, you still need to constantly check it and the frame you are building for level and square. You can always shim to level, but if your jig is not stiff in torsion, welding up your frame could pull it.
FYI, I met an ex machinist who built an absolutely beautiful Triumph powered special build bike. He said he built it on a piece of 3/4" plywood, reinforced to keep it from twisting.
Tom
Tom