Need help identifying heads

[ForcedFirebird]

Can someone tell me how to differentiate between FWD 3100 and 3400 heads by looking at them? Here are some heads I found on e-bay and they both say for 3.1/3.4 but I know the chambers are different sizes.
These appear to be the 3100 heads...
http://cgi.ebay.com/ebaymotors/GM-3-...QQcmdZViewItem

And these appear to be 3400 heads...
http://cgi.ebay.com/ebaymotors/GM-3-...QQcmdZViewItem
Could someone enlighten me?

[Superdave]

depends on the years... '00+ 3100 and 3400 top ends are identical aside from the plenum having a 4 instead of a 1. The older 3100's had a different intake port shape, the combustion chambers should be very similar.

If anyone has a casting number list that would help. I'll check the 3400 heads i have in the shop tomorrow...

[ForcedFirebird]

I'm looking for the 3400 heads with the larger chambers. If you look at the pics in the links, the two chambers are clearly different. One looks more like my RWD iron heads and the other has the "hearts".
Casting number on the "heart" heads (suspected 3100) are 24507487
And the others are pulled off a 99 but can't tell from the pics if the gaskets are just making the chambers look bigger, I'll see if I can get a casting number for the second set.

[SappySE107]

170 and 487 are the "large port" head castings. Large port meaning they also got the 1.76" intake valve. 99 transport is the 3400 minivan so it should be 1.76 as well.

Can you tell the 3100 (1.72) vs 3400 intake port difference? It was 1mm in height so I don't know if I could tell from a pic or not.

[SappySE107]

What? They are the same head. They all have the same combustion chamber for gen 3 other than slight machining upgrades over the years but the ebay heads look to be the same type head.

[ForcedFirebird]

What? They are the same head. They all have the same combustion chamber for gen 3 other than slight machining upgrades over the years but the ebay heads look to be the same type head.

That was one of the first questions I asked in the thread "head swap". I guess that is the answer I have been looking for all along. So there are no differences in the combustion chambers from 3100-3400? So the 99 heads are a good buy (assuming they aren't cracked/warped).
EDIT: Looking at the second link again I may have mistaken the mrks from the gasket as part of the chamber (making it look like my iron heads).