Small Port to Large Port heads?

[GrandestOfPrixs]

Hiya, first post here so my apologies if I don't format this properly.

I have a 95' 3100 V6 out of a Grand Prix, both my mains are leaking so I am deciding to pull it and rebuild(195,000kms). But I also want to do some power mods. My target is 400-500 Horsepower N/A, I've gone over to WOT-Tech and picked out some parts that would allow me to get there(except injectors) But I don't think using the 3100 small port heads will allow me to get there safely. Therefore I am wondering if I can put 3100 Large port heads or maybe even 3400 heads and intake manifolds on.
Would this cause any large problems fitment wise.
And another thing is since it is a 1995 Grand Prix, it has an OBD1, what type of fuel map tuning can I do on that? Or can I get a ECU out of a 1996 Grand Prix and use that?

[SappySE107]

400 NA hp is a lot of custom parts if are you sure this is your goal. You can put 3400 parts on your block and would be the way to go if you need to keep it a 3.1l. You cannot put a 96 computer into a 95 car without a lot of wiring and headaches, which will get you nowhere in the ability to tune. OBD2 is awful and 94-95 3100 OBD1 requires software I have no experience with. Even a 97 or newer computer swap would be the last choice I would take. Any aftermarket computer is going to be a better solution.

[GrandestOfPrixs]

400hp is the goal, not something I have to achieve. I am rebuilding the engine regardless, but what type of power do you think I will be looking at by just putting on Ported 3400 heads, ported lower and upper intake, and a stage 2 cam kit? And is there any other "simple" things I can do that will increase power?