Not totally true.
The 3.4 head has a better and larger valve spring, to handle the higher revs of the 3.4 cam
GM Parts Performance Book

Valves sizes.
S-10 heads (to 1985)/Camaro/Firebird (to 1984!) are the smallest valves of the 60* cylinder head series.
The 1985 F-Body & later 2.8/3.1/3.4 all use the same sized valves.
But in 1993 to 1995 (3.4) they got the really good heads.
Flow is the same all thru the years, tho.
None "better" than "others"
Just change in valve size then eventually better valve springs.

 

KED85, I can assure you though, that the heads on the 1985-1988 V6 Fiero are the 3.4 HO heads. The 2.8 Fiero V6 STOCK, put out the same horsepower and torque as the 3.1 liter did stock in the f-body. Not that this really matters for this discussion...

 

Beg to differ, 100%.
Why?
The Fiero used the East West V-6 engine & those Alum heads. Or Iron version of the FWD engine heads. The FWD head used on the FWD vehicles & the Fiero is the better 60* head design, that's a given.
F-Body ONLY used RWD Iron heads and that's that.
IF the Fiero Aluminum/Iron heads had the "larger" springs & such (which I can bet it does), that's a moot point.
The Fiero heads will NOT work on the North South V-6 engine.
Fit, yes.
Mate up to the removed 3rd Gen MPFI unit?
Nope.
I was given a set of the FWD engine aluminum heads. I threw them back.
They won't work on the RWD engine.
Unless you recreate the RWD MPFI manifold to mate up to the FWD heads.
Email me & I'll send ya a pic of the heads.

Early lo-po heads had 1.6 (I) & 1.3 (E).
The Hi-Po heads & ALL MPFI heads used the
1.72 (I) & 1.42 (E).
GM Performance Parts Book

No variation on the valve size.
The 2.8/3.1 & 3.4 head difference is the valve spring size and the spring pocket sized of the 3.4 heads.

Part numbers for the 3.4 valve spring stuff.
#12363215 Used on the 3.4 heads-Valve spring
#12363216 Used on the 3.4 heads-Retainer used for larger 3.4 sized springs. Diameter is 1.203" & the stem used with this has a 11/32" stem diameter.
GM Performance Parts Book.

The 3.4 head is different/better, in those few ways.

 

See, KED85, that's the thing, the Pontiac Fiero V6 heads are ALL Cast Iron, they never made an aluminum block or head in any Fiero ever. Even the Iron Duke / Tech-4 4 cyl motor in the Fiero is all cast iron.

The engine IN the Fiero IS a FWD V6 motor, but it's the exact same engine block as found in the RWD Camaros. EXACT same.

As a matter of fact, the lower intake, and runners in the Pontiac Fiero are ALSO the /EXACT/ same pieces used in the 3rd gen 2.8 and 3.1 V6. EXACTLY the same. The intake manifold that I have right now on my V6 Fiero was from a 3.1 V6/60 in a late 80s f-body that I pulled from the junkyard. I wanted to keep all the "numbers matching" pieces in my Fiero original and UNTOUCHED.. so when I wanted to port match everything, I got a lower intake and runners from a 3rd gen.