I'm just about ready to paint my 70 Chevelle and I'm looking for some advice. Should I remove the fender extentions and grill and have them painted separately or have them painted in place??

If you haven't disassembled the rest of the car, then leave them in place. Depends on how much masking you want to do.
My .02!

Remove the fender extensions and the grill. If you paint the fender extensions "in place", you run the risk of forming a "paint bridge" between the fender and the extensions. Also, at this time, it's easier to paint the grill "off the car".

You might want to consider loosening the extensions, space them out from body, and spraying in place on car.

Have to be careful of dirt getting blown out from space between extn. and body. Be sure to blow area several times before spraying. However this does help to prevent paint bridging referred to earlier.

Always remove grille, spray separately, cuts down on masking and overspray.

Karl