Two Different Heads?!

While helping a friend tune up his '78 Trans Am I noticed it had two different kinds of cylinder heads. There is a 6X on the drivers side and a 7K3 on the other side, I didn't know this would work since they have different size valves and probably other differences. The car runs suprisingly well and doesn't seem to be affected, but I've never seen a 400 done this way. I'm wondering if makes a difference when tuning it up - spark plug gap, carb adjustment, timing etc.?
It's a daily driver - mostly stock with no emmission control items.  

The 7k3 has the same intake valve size but larger 1.77" exhaust valves. It also has smaller 96cc chambers. So essentially, the side with the 7k3 is making more power than the side with the 6X, due to it's higher CR and larger valves. Unless the 6X is the "-4" from a 350 or a W72 400 with 98 cc chambers, which would put the CR very close from side to side with only the exhaust valve sizes causing a slight difference and then only at higher rpm.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Would the casting number say if it's a -4?  The only other difference I really see (besides the exhaust valves), is just a few extra ribs on the top to route the oil.
Either way, I guess it doesn't matter much if it's not used for racing or shows.  

There will be 4 or an 8 stamped, not cast, into the head above the end ports near the valve cover. The "-4" was used for the w72 400 and the 350 and used smaller chambers, the "-8" were used on standard 400's . Wallace racing lists 6X-4 as 91cc and the "-8" as 98cc, but I recall the actual measurements are more like 98 and 101 respectively. Either way, you can see the difference in the CR on each side of this engine. For a real performance buildup, you would definitely want to use matched heads for smooth balanced power and long life. As for tuning, there's not much you can do as the dual plane intake design feeds criss-cross from side to side and you can't alter timing from cylinder to cylinder.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Got ya, I'll forward your info - thanks as always!
Brazil  

Steve, I want to make sure I'm looking at the right place for the number. The 6X head does have a big "4" with an "M" above it on the end exhaust port, and the 7K3 head has an "8". I may be getting stamped confused with cast but that's the only reference I see to those numbers. It's large raised letters just like the 6X.
Slowly learning, thanks again.  

The secondary stamping is on the rectangular part that runs vertical.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Is it raised letters? I don't see any small stamped in numbers.  

What would the 4 with an M above it mean?  

Also, is it on the flat end of the head? I may be missing it because of the power steering pump bracket.