I have a 71 cutlass conv. and just removed the fwd inner wells.
They are in good shape, but after I cleaned the dirt off, they
are lightly scratched and kinda off black in color.

Does anyone know a good way to shine up plastic without painting.
I was thinking of lightly sanding up to about 1500 grit, but not sure.

 

There are lots of detail products out there that shine up plastic well. You might even find a paste compound for polishing plastics if you go to a place that specializes in plastic products. Check in the local phone book and make some calls. Since you have them off the car, now is the time to do it. You might try the sand paper approach on an area that you know won't be visible when installed (maybe the inner side, back, facing the firewall down low). I haven't heard much good about the armour all products lately, even read about a claim that it is harmful to plastics over time. I will say that I don't think it lasts very long.