It's pretty hard to mess up a huddleston with weights, which is really a testament to how well balanced the lure is.

Anywhere along the belly should be fine if you want to add weight. The lead harness makes it hard to put anything in the front 1/3 of the lure, but you can slide weights in parallel with the belly there if you want. If you can't get enough weight there, just go a little behind the harness, shoving the nails straight up in to the lure.

If you use a pair of pliers to push the weights most of the way in, you can test the lure with different amounts of weight, then - when you get it right - push the weight all the way in and put a drop of super glue gel.

are good for this sort of thing. The lead they use isn't particularly heavy, so you have to use a fair number of them, but they are very cheap, and the little edges on the nail keep the weights from falling out (most of the time).