You can do the doors by weighing all it's pieces and then put a sand bag the same weight inside the door shell. Hard to figure the weight for the rest of the car, but, you can make a guess on where to to add weight where it will effect the body flex. With the car on it's feet, that shouldn't be too hard. Off the ground, I'd have stands under the rear axle shock plates and front lower control arms out by the ball joint. If it has no interior at all, no engine, trans, bumpers, trunk lid, etc, I'd get some buddies to add weight to see how things change. Maybe a couple sitting in the engine compartment, 2 more inside the car, and one in the trunk would be a good start. That should give you an idea of how the gaps will change once the car is assembled. Naturally, it would have been better to have replaced the parts before disassembly and set all the gaps then, but, sometimes that just isn't feasible.
