I know Marxist are statist. That's why I'm saying it would be far more interesting for you to have the anarchists and communists in Spain to come together under one banner of true freedom. They could have the opportunity to create a truly free society. I have no clue how you could achieve this but I'm sure some well to do propaghanda, the evil of fascism, the struggling economy, and the disaster of the USSR. Could help institute this.
Setting aside what you understand for "true freedom" (for a start, Communism in that time meant Stalinism for most of them, and they were responsible for most of the atrocities in the Republican side), this is absolute ASB. There's no way you can alter events so Anarchists and Communists stop hating each other to the core. The Anarchists were as anti-communists as anti-fascists. You are deliberately ignoring the post-2nd International political climate, and that in Spain, anarchism had filo-Trotskyst sympathies, not without a reason. And even if ASB technology brainwash them so they cooperate, they're still not strong enough to overcome the Republic, let alone the Nationalist side.
Then again, I guess there's a way you can have an Anarchist Spain, but it would take a pre-Civil War POD, without any doubt.
As for the WI, then the Anarchist would be crushed by the Republican government, which would divide the Republican effort and guarantee an earlier Nationalist victory. The long-term effects would be mostly limited to Spain, and they wouldn't be very drastic unless some key Francoist figure gets off the equation.