Looking great, my friend @yann!
In order to find the correct ballast, an approach would be to assemble and attach the basic components (wings, tail etc), in order to have the basic model assembled. Then, let the model balance on your pointer finger: you will more or less see where the center of gravity lies. Then, add ballast to the points youn plan to attach (securing it with modeling clay or suitable means). Add as much ballast required, as for the center of gravity to be positively in front of the main wheels touching axis.
Please take into account that at some planes. like the Liberator, the main landing gear legs are inclined to the front, so the main wheels touch the ground further front than the point where the main legs attach to the fuselage.
Also, you must take into account the weight of some components that will be attached at later stages, (like props, seats, guns, clear parts, whatever…) and vary the ballast accordingly as you install it at these early stages.
Finally, of course, better be safe than sorry, meaning adding a bit more weight than required, BUT please note that puting TOO MUCH weight will probably stress that scale looking nose gear leg…..
Looking forward to your progress!