The numbers are a lot easier to make sense of if you write them like this:
LF 343 + RF 313 = Front 656
LR 321 + RR 330 = Rear 651
Left = 664
Right = 643
LF + RR = 673
RF + LR = 634
So, your car is heavy on the left. A heavier battery, or lighter electronics, or moving your battery out, or moving your electronics in, would do well. But since you have to add weight anyway, you can just do that.
Your car is pretty balanced front to rear, actually a bit front heavy. I haven't experimented a ton with front/rear weight balance, but I usually run more weight to the rear. Rear corners about 10g heavier than front corners. More rearward seems to give the car more steering in the first half of the corner, but less in the second half of the corner. It also seems to be a bit easier to drive. Going too far back seems to make traction roll worse, though. In any case, I'd start by adding weight on the right rear.
Also important to note: because the left side is 20g heavier than the right, and the front/rear is roughly even, you'd expect to see the left front ~10g heavier than the right front, and the left rear ~10g heavier than the right rear. That is, you'd want LF + RR == RF + LR. But what you see is a ~40G difference, meaning either the scales aren't sitting evenly, or something is fairly tweaked. You'd probably notice that amount of tweak on track. That can come from a lot of things. Assuming the scales aren't sitting really unevenly, the most likely causes are tweak in the chassis (top deck most likely), uneven spring lengths (common on associated and Schumacher springs), and tweaked swaybars.
For getting a good reading off the scales, you'll have to experiment to see how your car and scales react to different ways of settling the car. You can try lifting the car and letting it settle back down. You can try pressing down and letting it come back up. I usually end up with a combination of gently tapping on the center of the shock towers, and rolling forward/back a bit to get it settled. The readings can vary quite a bit (like 5-10g per corner) based on how you get the car settled. So you just need to play with it and find what it likes to give you the most balanced and consistent readings.
From here, I'd suggest the following steps:
- Play with orienting the car differently, spinning the tires differently, moving scales around, and different settling techniques to get a feel for how reliable your readings are.
- Back off your droop screws a bit, so that droop isn't affecting the readings.
- Put on some setup wheels if you have them.
- Disconnect the swaybars
- Go ahead and add the weight you need to add
- Play with weight to get total left/right weights the same (within ~5g is close enough). Don't worry too much about cross weights just yet.
- Now take the car off the scales, and re-set your ride height and droop. Get the droop spot-on using a droop gauge and blocks, or blocks and measurement off the setup board.
- Put the car on the setup board with tires on
- Lift the car slowly from under the center of the rear with a wrench, and watch the way the tires lift. If the left tire lifts first, tighten the right front spring slightly, and loosen the left front the same amount. If the right tire lifts first, do the opposite.
- Now spin the car around and do the same thing lifting the front, and adjusting the rear springs.
- Put the car back on the scales, and your cross weights should be very very close.
- Reconnect swaybars one at a time and make sure it's still good. If connecting a swaybar throws off the cross weights, de-tweak that swaybar.
- Re-check ride height again, and then re-check with the tire-lift technique.
I prefer to use scales to get the car balanced the way I want, and for sanity-checking tweak, but I use the droop + lift technique to de-tweak. Whenever I try to use the scales to de-tweak, I end up going in circles with the spring adjustments, and sometimes end up with really weird adjustments, because the scales don't tell you which end of the car the tweak is coming from. The droop + lift technique is really simple, and it's closest to what the car is actually going to be doing when it's on the track, which is what really matters. However, that technique doesn't work at all if the car's left/right balance is off, because it'll look tweaked on both ends, and you'll make some screwy adjustments. It also doesn't work if your droop isn't spot-on using blocks. Sometimes I'll lift one end of the car when it's on the scales to try and figure out which end is tweaked, but even then it can still lead to some weird adjustments.
Some people may disagree with me, but my recommendations are based on tips and tricks I've picked up from top-tier drivers along with my own experience of what works to get the car to work properly and consistently on track. There may be other things that work, too. There are also some factors (like actual symmetry of weight placement and its effect on chassis tweak) that I just don't worry about, because I usually can't really do anything about them. The bottom line is to understand what assumptions you're making with different measuring techniques, and making sure you get the most basic measurements and adjustments right first, before making down-stream measurements and adjustments. Otherwise, you'll get things screwed up pretty quickly and have a car that you think is set up correctly, but drives like a bag of poo. The absolute most important first thing is chassis tweak. The chassis has to sit flat on blocks, because if it doesn't, the droop numbers will be off. Then the droop numbers have to be spot-on, because if they're not then my suspension de-tweaking will be off. Etc, etc....
Also, if your club has an official scale for tech purposes, be sure to weigh your car on that. Our club's scale reads a good 15-20g lower than my 4 combined scales.