Yep, the solid rod you're referring to is called a Ferrule
, you'll use the 2.5" for the leading edges and the 4.0" for the spreaders. The longer length used on the spreaders is to allow for the width of the Center T, when replacing the spreaders you won't center the ferrule in the spreader like you would on the leading edge, you need to factor in the width of the center T so the insert point will be 2" - (1/2 the center T width). Hints:
Wrap some masking tape around the ferrule at that point before gluing & inserting it to act as a stop to avoid pushing the ferrule too far in, it never fails if you push it too far in the glue will bond instantly at the wrong point. It also allows you to insert it quickly with confidence so the glue doesn't bond before it's in far enough. The glue goes inside the spar, not on the ferrule, if you put it on the ferrule you will get a buildup of glue where the rods meet that may cause the female rod to split if you don't get it cleaned up immediately. Check that the ferrule will insert into the rod before gluing, once in a while you'll find there may be a burr or irregularity that will hinder a smooth insert. Clean the ferrule and inside of the spar with a mild solvent or rubbing alcohol to remove any release agents for a good bond, some people recommend using some 400 grit sand paper to get a better bite.
If you ever need to replace your 2PT's be sure to measure the diameter, they have been available in both .244 inside diameter & .234 I.D. currently I believe only the .234 ID is available in the 32.5" length.