I'm back working on this project. After hearing some suggestions here, I've abandoned the linear actuator approach, and taken the suggestions. (Thanks to those who made them.)
Instead of linear actuators, I'm planning to use the existing steering linkage and gearbox for the left wheel. I'm going to disconnect the existing steering linkage for the right wheel. To turn the right wheel, I'll go to the junkyard and get another steering linkage and gearbox. So I'll have an independent steering linkage and gearbox for each front wheel.
I'll need two motors to turn the gearboxes. I'm trying to find motors that would work. As Soeren says, they should be heavy duty. They'll need to turn the shaft that the steering wheel turned before. If I recall correctly, that's about four revolutions lock left to lock right.
Any suggestions on motors? I would like to scavenge them from something, but I can't think of what other device might have a motor that would work. Maybe some sort of forklift?