Okay, so its only .4in larger, so why don't we just use our revision to make a direct fire anti tank weapon.
We cant use our design phase to counter their previous one every year, or we will lose, albeit slowly, we need something that will put them on the back foot.
A flamer will let us stop them in the trenches, torching tanks, especially if we get a revised AT gun from the bombardier, and we can thrash them in the jungles.
We want a self-propelled weapon, so we can respond to assaults, and push the enemy back! Stationary weapons are only good for defending. We can't attack if the guns aren't mobile. We can't destroy enemy emplacements or fortifications. Your way leads to stagnation, not advances. I should also point out that if the gun is smaller, it will be less likely to damage bunkers, if the enemy ever develops concrete fortifications.
That's what this mobile AT will do. You underestimate the advantage their armor gives them. It NEEDS to be countered, and in such a way that will lead to further advances. Grenades can't help us destroy emplacements because of their short range. You'd never get close enough to knock out a machine gun with it on the plains. That's what the SPAT will do.
Flamethrowers are just like the grenades. Defensive weapons. We can't use them in attacks on the plains because the men using them are vulnerable to machine guns. The SP-AT will lead to breakthroughs in the jungles and mountains as well where the enemy is entrenched, because we'll be able to destroy their strongpoints.
E: The solution to turning things around certainly isn't to ignore their advantages, instead of countering them.
E2: If we develop this tread platform now, we can put a flamethrower on it later.