Technical Fenton header question

I recently bought a set of Fenton headers for my Mopar flathead 6, and am designing the exhaust system for it. Realizing I need 2 flanges, I discovered the headers have different spacing on the studs; one is 3-1/8" and the other 3-3/8". Was there a reason for this? The headers are a matched set H200 & H201. Why would this be different? Just saying...

 

Appears right bank has been opened up 1/4" over left bank. It then appears to step down to 3 1/8" from 3 3/8" where the ports intersect

 

Why are the output holes in the headers so much different in size? Something is very wrong there.

 

not familiar with mopar parts , but are you sure those are exhaust headers? my guess would have been intake. what would be the reason for those two pipe plugs?

i really don't know what i'm talking about here , so i'm probably wrong

 

Do you think that is has something due to balancing the front three cylinders to the back three cylinders.

 

I expect it was designed to be used with a y pipe, not duals. The large one being the main exhaust tube, the small one joined into the larger one. Just a guess.

 

My question is mainly about the spacing of the studs/threaded holes. I believe the smaller outlet exhaust port might have something to do with balancing out the back pressure, I dunno. All cast Fentons I've seen for the mopar 6 flatty look like that. Here's another set...

 

The big one fits the factory headpipe, the smaller dual side is Ford flaphead size for ease of matching when the headers were sold without the complete dual set.
The plugs are for heat riser plumbing - it rotted away the first year if installed.