Jakarn' These neck nubs are molded from legacy figures and are slightly larger than the ones on the clone wars figure bodies.
To make them work with these cw heads, all that is required is to ream out the lower edge of the mounting hole in the resin head with an exacto knife. Not much material is needed to be removed and this process doesn't take very long to do. In the example I have packed into your free parts bundle, I have left the fit fairly tight from the nub to the head. Rather than simply gluing the nub onto the cut off post of a figure's body, I would also drill a hole into the cut off post of the body and into the joining surface of the neck nub so that you can use a length of wire to strengthen and pin the connection point where you glue the parts together
While the Echo heads fit perfect into the Echo helmets, I find that the Rex and Gree heads are slightly too tight in the nose and ear areas. A simple way to counter this is to slightly widen the opening at the bottom of the helmet with an exacto knife and to slightly carve out the area inside the front face of the helmet where the head's nose resides while in the helmet. This mod is fairly easy to do and takes little time.
In the last pic, you see the slightly altered echo helmet with the Rex fitted inside and the nub fitted to the Rex head in the way that they are included in your free parts bundle. Use this set as en example for any fitting mods that you might need to make.
I am packing your parcel now and will ship it within the hour.
Thank you so much for your business and, enjoy your parts.