3400 DOHC? May you be reffering to the 3.4L DOHC
GM has three 3.4L motors they used and people do get them mixed up a lot. I was thinking of doing a 3.4L DOHC at one point in time, my idea was have three fieros and each one have a different 3.4L motor in it.
I know one owner who had a DOHC motor for the longest time and was trying to sell it. He's since moved to London, but I know his Fiero is still in Durham, so the motor might be there as well if he never got rid of it. The swap can be done, a few members on Pennocks have done this swap. It would be wise to build it from scratch so you know the block is good. Little more modding in the engine compartment to make it fit with those big heads on the motor. 205HP stock, little cam adjust will bump that up.