It should shift on its own if there's enough fluid. If it's a 350 it will have a kickdown cable. If it's a 400 it's a switch. No cable, I think. Either way, they're not necessary to shift. If there isn't enough fluid, it may not engage into first and will have trouble shifting from there. Run the motor for a few minutes now that you can Add fluid a 1/2 or full quart at a time until you get a correct reading on the dipstick. Make sure you do this in park of course.

P.S. I am pretty sure both types require engine vacuum as well. This connects to the modulator valve on the tranmission.