Have fun .
I think that might be one of the worst kits Tamiya ever made .
BTW , the plane being designed around the gun is one of those unkillable myths but it's still a myth .
I think the like every rumor, myth, or legend there is a fleck of truth some where in it.. The plane may not have been built for the gun but the plane was designed to carry that gun which was a engineering master piece with in it's self.
CAS close air support was what the military was looking for, they needed a plane that can pack some fire power, loiter (stay in the AO) longer then the average fighter jet, fly lower, and handle combat damage. The A-10 fit all these roles beautifully.
The GAU-6 Avenger cannon weighs in at around 600+ Lbs with out ammo or the feed system which when added brings the total to roughly 4,000+ Lbs. Which combined with the size of the gun and it's components leaves very little space to put it on a airframe capable of filling the CAS role. Size aside weight and balancing for a fully loaded GAU6 can't be just any place on the aircraft it would have to be dead center line and forward to balance the weight of the engines needed to provide the thrust needed to fly the aircraft and it's payload. The pilot pretty much sits on top of it. Imagine that seen from Armageddon we're Steve Buscemi rides the nuke scream about the power between his legs.
And since each round of the ammo for the cannon weighs in at 1.5 lbs and the rate of fire is 2000-3,900 rounds per min means that center of gravity changes rather quickly and the plane needs to be able to compensate for that.
In my small 4 Seat piper warriors I can technically only seat my self and a friend with a full tank of gas and if that friend is heavier he may have to sit in the back instead of the fron or the plane won't fly right the A-10 has to deal with this on a whole different scale.
Then on a totally different front you have to consider the recoil from fireing the gun at a sustained rate which even though doesn't greatly effect the plane does play a small role in the equation.
Around the time this plane came about or maybe just before the air force was looking for a 30mm cannon design and this gun was the winner so yes the plane wasn't built for the gun as the myth goes but it was definitely designed in a manner that had to make it capable of carrying it.
So there always some truth some where in the myths that we hear in the military no matter how big or how small