Thank you, kind sir!
Obviously, I'm just repeating things already done and adding what seems like productive methods of doing it again.
The Chrysler 218/230 look like the best candidates for actually making some of the parts, since they're the most popular L6 flathead engines and they're pretty durable AFAIK.
I'm sure a casting (like Edgy did) will work very well, but I tend to favor things that I can understand and control better, and making a "module" out of commercial shapes looks possible.
Use 2 90° Schedule 80 elbows (90° is, IMHO, a good compromise between flow, height, and carburetor fit since down-draft will make everything taller) for the port wall and ID of each module (total of 6 elbows, 3 modules), seat them the same distance apart as the intake guide centers on a piece of 1/2" plate.
The plate should be bigger than the bolt pattern around a pair of cylinders (that share an intake port: 1&2, 3&4, 5&6) so that these bolts (replaced by longer bolts or studs where needed) can be used to attach the module. The elbows must be rotated toward each other (the exact angle depends on the center distance, and how far the stubs protrude laterally) so that the ends can be joined to make a siamese port. It may be possible to use an actual (Offy, Edmund) 3 × 1 or 3 × 2 manifold if the port center distances match the original - saves buying a manifold!
Weld the elbows to the bases all around the exterior.
The commercial valve seats go underneath, tilted at an angle favoring the bore center (this is also the angle you want for your donated rocker arm, so pick a V8 etc. rocker with this valve stem angle).
Now the hard stuff: the guide boss has to be part of the roof of the port. A 1" cylindrical chunk of 6061 with a .625" hole in it (you need 6 pieces) can be placed in a hole at the stem angle in the port, aligned with the seat, and welded in place on the top only (it only sees vacuum - no pressure, no water, no oil) to make welding easier. The hole fits a 5/16" ID commercial valve guide (bronze = works with less oil?).
The rocker support will depend on what you want it to look like. A single piece could house all 6 rockers and span the length of the head, but looks weird. Individual rocker housing could top each elbow at the guide boss (2 per module), but it's more work. Having a pair of rockers in a single housing (1 per module) is a good compromise.
