We've had this debate before, but it doesn't seem like we have progressed very far.
If you choose just the right sized spheres and can get them to pack in the most optimal way (= face-centered cubic close packing), you'll have 74% solids and 26% voids. If your packing is less than ideal, you might end up with only 65% solids, or quite possibly less.
If you add some finer particles, you can indeed fill some of that void space, but you run the risk of disrupting the ideal FCC packing and negating much or all of the gain. And, as Tony says, the fines have pores spaces too, and lots of them. Assuming we are talking about something like the interior space of a center-sill, you're not going to be able to apply a ton of pressure to get the particles packed, and even vigorous tapping of the underbody may not work because you'll just bounce the shot out of that space.
So, practically speaking, what's the best you can expect from mixing "big" shot and little particles? 80% solids? 85%? If the latter, you've increased your added weight by a mere 15% compared to FCC-packed uniform spheres. All else being equal, if you switch from lead to tungsten (the metal, not the putty), you can increase the added weight by 72%, which is starting to sound like it might be worth doing.
Combining the above with Ted's sugar cube example, the cube will work just fine if you can trim it to a shape that fits snugly inside the center-sill. Practically then, you could just take some thin lead flashing, cut it to the correct shape, and stack it neatly inside the sill. With a little care, filling ~80% of that space should not be at all difficult, and that's about as good as you are going to do short of casting a solid metal piece that exactly fills 100% of the space.
PS: I can't get that excited about tungsten putty, which contains about as much polymer as it does tungsten. At a density of about 10 g/cm3, it only would be marginally better than my 80% lead example above. Tungsten powder has more promise; at ~70% solids you should hit ~13.5 g/cm3 versus 11.3 for solid lead. If you can get it to pack that densely.
Swall Meadows, CA