After digging, it seems like the matched set 72-73 340 manifolds (and others?) Have a 2 1/4 outlet on the driver's side, 1 7/8 on the passenger side. Is there a matching 2 1/4 passenger side, or does the difference really matter?
My 340 manifolds are what you described. I was using a stock exhaust which necks down to about 1 3/4 or so at different places, so I decided it doesn’t matter.
If you search the forum, sone folks use jeep Cherokee passenger side manifolds the can be modified with a little grinding to be 2 1/4 as I recall. However, the outlet is is a different spot, so you will need a custom head pipe.
Does the 1 7/8 make a difference? Probably does, just depends on how much time and money you have to chase down the rabbit hole……