LS3 VS LS7 Heads
 

What is the difference between LS3 & LS7 heads. Can you use LS7 heads on an LS3.

Differences

Slightly larger valves:

LS3: 2.16" intake, 1.60" exhaust.
LS7: 2.20" titanium intake, 1.61" sodium filled exhaust
(These might present an issue with piston relief areas, depending on valve lift used, not sure.)

Combustion chamber volume:

LS3: 68.7 cc
LS7: 70cc

The LS3 uses a rectangular shaped intake port that is a bit taller than the LS3's port. This can create some minor flow issues if used with the LS3 intake manifold.

Fit

The bolt patterns and other features should be the same on all LSx engines (I think that is true) but you have to be careful to match intake manifolds, and the slightly larger chamber volume will change your compression ratio just a tad, although not severely, and may also present some flow issues because the combustion chamber will hang over the edges of the bore just a smidgen... probably not a big deal, tho.

Those are the only major differences I'm aware of.

Intake runner volume is greater on LS7 and suited to a larger displacement engine

Doesn't the LS7 have a D shaped exhaust port, while the LS3 has the LS2/1 style round exhaust port?

NO.
Minimum recommended bore for the ls7 head is 4.10

I've heard the opposite to be true suprisingly(ls3 ~260cc/ls7~245cc) Someone correct me if I'm wrong...

No, I believe the exhaust port shapes are the same

LS7 = 270 cc (from GM performance parts catalog)