Godzilla......
Hmmm. A lot will depend on the model you're using...
Size principally controls lethal range, so Big G's size should be equal to the distance in inches at which you feel the model could hit someone by kicking, clawing and/or tail lashing WITHOUT moving from the spot it's standing on.
That also defines what he can hide behind.
Move....whell, it won't be cinematic without a chase scene between Godzilla and the heros - desperately fleeing in a jeep through the city as the big guy stomps along in their wake. This means a HMMV should be able to run away but not get away, or only get away slowly. Since a HMMV has a top speed of 15" per turn (Wheels/15), then a move of 7" seems about right.
Close Combat. Definitely D10's, and should be able to hurt a tank with an average roll, so D10+3 or more. I'd go with the lower value and add Killshot and Multihit to his traits - that means a tank that's hit gets smashed outright but there's still a vague chance of not inflicting the hit in the first place (someone dodging, etc). Jaws + 2 claws + 2 feet + tail adds up to 6 dice, in my book.
Target of8+ is ok...remember that as long as he has armour 2+ and a million or so hits, it doesn't matter if you hurt him; you'll just make him angry. Whilst it should take a lot to bring him down, an A-10's main gun should at least be able in theory to hurt him.
Armour - 2+. just because.
Kill should be high enough that it's only attainable by something really big. Remember that this isn't "you're dead" but "blow a big chunk out of his side and stagger him" - something accompanied in godzilla films by a lot of pyrotechnic squibs going off and godzilla falling on a skyscraper. Make it more than a tank but not too much more - so a dedicated AT weapon can still get it on a 10 or so. 13+ sounds ok.
Traits
Killshot
Multihit
NOT lumbering - Gondzilla is a living creature and hence should be able to react to things.
Hits/lots*
*this is going to depend on how much stuff you want him to hammer his way through; 10 is a good start but obviously won't last that long against a 18,000 point armoured battalion, whilst 15 is going to make a game against a 1,000 point infantry platoon so one-sided that it probably won't actually be fun.
Remember, it should be possible to drive him off (obviously you can't kill him, but at Hits/0 he's hurt badly enough that he'll leave), or the game is a totally forgone conclusion. But it shouldn't be even slightly easy....
Radioactive Breath
Looks good. Ready and Slow seem right as there's the whole 'lightning crackling up back' thing he does first.
Lethal Zone/6 might be a bit much - not that it's wrong for Godzilla but there should be a difficult choice between flaming stuff and running up and hitting it with your tail*. If you can destroy a 12" diameter swathe of the enemy every turn, who needs to smash stuff?
* Now I have a mental picture of Isolde from Ozy & Milly doing Godzilla impressions....
