i got a hatch cover for like 3 bucks at a u pull it place off a firebird. now will it fit with a hatch pull down motor type lock? is the back plastics diffrent at all?

You should have grabbed the rear most plastic--it is different from the non-hard cargo cover cars. It has a small raised lip w/a notch for the locking lid.
It also depends if the donor car and your car has a pwr hatch motor or not. The hatch motor means the rear panel has a larger raised area(hump) then the non hatch motor cars.
This is the one that belongs w/the hard cargo cover.(hatch motor)
Can you see the difference from yours?

This one is the non-hard cargo cover panel.

There's also some additional bracing that holds the back panel up, and provides a place for the lock catch, IIRC. I cut those out of my donor car with a chisel, and just popriveted them to the tailpanel of mine.