updated 01 mar 2020
The exhaust side of the 196.6 OHV head isn't too bad. though the hot gases pass through the head for 3" or so, with attendant opportunity to heat the head, the entire path is a straight shot into the manifold, after the usual 90-degree turn in the pocket. the port is sort of rough, but large and rectangular.
the six cylinders exit the head as three port pairs, 1-2, 3-4, and 5-6. 1-2 and 5-6 are identical, a straight shot from the head into a manifold.
the center port however, cylinders 3 and 4, has a huge chamber and odd shape that directs exhaust gas upward, onto the floor of the intake trough, to heat the mixture out of the carb. Overall the center port is twice as long as the other two, if the gases indeed run up to the roof/intake trough floor.
I assume (umm, not really backed up by any facts) that unequal exhaust ports is not helping performance. whatever happens to gas flow in a port cannot be helped by the abrupt U-turn and probable turbulence in the center port. Carb heat is not really an issue in a trough type intake! Seriously, the whole thing is solidly at head coolant temperature. (Though I did actually measure intake charge temperature at speed, it was interesting.)
[01 mar 2020 note: Around 2010 I drove around with a thermocouple mounted inside the plenum to measure intake charge temperature, to dispel a bunch of assumptions, etc. I seem to have lots the page with photos and actual numbers. It is very simple: evaporating liquid (gasoline) in the carburetor takes a lot of energy and the charge temperature drops as much as 50 degrees F at wide-open-throttle. Silly devices like the heater water pump mounted to the trough plate do literally nothing to charge temperature or density. It probably keeps the carb warm-ish in very low speed driving in very cold weather. But it cannot have any effect on the temperature of the intake charge -- it simply moves too fast to absorb heat. Waste of time. Conversely, it greatly lessens the effect of hot vs. cool air into the air filter. And the increase in air density (eg. any "supercharging" effects due to cooling) are measureably negligable. Any racer says otherwise? Ask them for measurements.]
But it turned out not too hard to make the center port have the exact same size and shape as the other two. The center port casting is very wide, to accomodate all the ducting up and down. I simply (yeah right) fitted 1/4" steel plate sections to box in the port. It really was luck however that the three pieces that comprise the box were able to fit so nicely.
The box sides have to extend deep into the manifold however to reach part of that casting of the same width of the others. This took a bit of tweaking on the bench to allow the manifold to be installed and removed from the head while in the car. It also prevented the use of the usual mounting studs; I substituted stainless steel hex bolts and washers.
The three plate box components were assembled within the port and welded to each other. They were not welded to the head in any way.
Here's the exhaust manifold installed complete, for reference.
Here is the small "good" 1 and 2 or 5 and 6 exhaust port. You can (almost) see that it goes straight in to the exhaust valve pockets. The head is upside-down in this photo (which is how I worked on it on the bench).
Here's the center port. The head is upside-down here also. Note the depth of the U-turn the exhaust port makes up to the intake trough floor; that's a 6 inch rule, the chamber is nearly 2 inches deep. This chamber will be blocked with the roof filler part.
I made heavy use of my sliced up cylinder head in working out these details, which has been very helpful in this project. I'm not sure if these pictures will be illuminating at this point, possibly they will help to refer back to later.
The work started in earnest by cutting cardboard to work up a basic approach. From these I cut rough steel parts.
The roof piece required the most work (recall that the head is upside-down here; the port roof is at the bottom). I trimmed it mostly with a small bench grinder. The innermost end of the roof filler I cocked up a bit with a slight bend, not really visible here. The roof piece is inserted diagonally then dropped into place. The notches in the piece are to fit around bolt-hole bosses.
With the roof more or less fit, the side wall plates were fit. These were easier in the head end of things, as they only had to reach the head bolt bosses. Things were a little tricker in the portion that reached into the manifold; more on this below. Too bad I didn't take more pictures here, it was basically a lot of iterative grind and fit, into the head, then into the manifold (both shown here).
Here are the three pieces before they got welded in place; front and rear. Note the build-up on back, and if you look closely, you can see that the side pieces were Frankensteined, chopped and welded, multiple times for a very close fit. The lumps on the reverse are to position the parts against the head casting.
The final step was to weld the three pieces to each other within the port. My welding isn't pretty, and I don't have a die grinder/Foredom at home, so it went in the car like this. Meh. The port dimensions are now identical all the way from the valve pockets to the manifold.
Here are some other views of the parts and port as installed. From within the valve pocket you can see the roof filler corner. From the outside, the side walls protrude, where the fit inside the manifold.
This is the view from the exhaust pipe end of the manifold. The center port now blends smoothly into the "Y" of the manifold; the two side branches for the other four cylinders come in cleanly from each side.
I made some changes to the manifold itself, mainly removing the choke stove, plugging the holes and keeping the internals smooth and clear. The top choke stove hole got reamed to a taper and fitted with a brass plug swaged into place. I sliced the brass plug flush in place with a hacksaw blade. (The Weber carburetor has an electric choke.)
The bottom choke stove hole got a bolt installed with the head ground funny to fit flush with the manifold walls.