Welding up cracks in 4-speed shifter

That’s going to take some work if you don’t want it to just crack again. It’s location in the corner at the flex point mean that fatigue, work hardening and crack propagation will have damaged a lot of the surrounding metal that you’ll be welding to.

Stop drilling it, grinding a groove for the weld bead, and then welding would be the best solution. But at that location the crack will still probably recur eventually. You can buy those shift plates separately and replace them in rebuilding the shift box.