Does the engine still have the intake on it? If it's a F-Body 3.4, the intake manifold should look like a big letter Y. If the intake looks like a spider, then it's a 3400. See photos below.
Also, I forgot to mention that the F-Body 3.4 actually has less horsepower than the 3400 V6. And as far as I know, there was no "less horsepower" version of the iron-head 3.4 V6. So if you accidentally got a 3400 V6, you're actually getting "bonus" horsepower. You'll also be getting a roller cam, reinforced engine block (stronger than the F-Body 3.4 block), and a reinforced bottom end. You also won't need to drill holes for the starter mounting bolts, because they're on the correct side. The downside is that if you want to use the Fiero heads and intake, you'll need to swap in 3.4 V6 pistons, because the iron heads on the 3400 engine would have really high compression with the stock 3400 pistons.
[This message has been edited by Blacktree (edited 04-29-2017).]