This response is a little off target, but applicable.
Over the years, I've heard of a few cars having one '840 head & one '291 head. Other than the spark plug seat, these heads are supposed to be the same ( functionally ).
As was previously mentioned, the '291 heads were available ( in service ) with the older style spark plug seats. As a matter of fact, the older seat style
was used on all aluminum BB heads throughout production.
There is an inherent mismatch from combustion chamber to combustion chamber on the same head. This is due to machining, casting, etc.. So, using one
'840 head & one '291 head shouldn't pose any real problems. As to the question of one of each being used on the same production engine, I would tend to doubt it.
I'm doing some research right now concerning manufacturing tolerances in the engine ( BBC in my case ) machining process. I have learned some startling information
about how sloppy the tolerances really were on our beloved engines. It has been reported that piston to deck height clearances ranged all over the place and out
by as much as 0.020 - WOW! When you add combustion chamber volume variations, actual piston design fluctuations, rod centerline-to-centerline variations; then throw
in intake runner volume variations coupled with exhaust port variances, you start to see the real picture. I was told that production engines were designed with these
fluctuations in mind. It was just a matter of manufacturing reality in those days. One very knowledgeable builder told me that an L/72 was pulled apart and upon
inspection it was found that the "as advertised" compression ratio of 11:1 was actually less than 10:1 - HOLY SCHNIKE-EES ! Another high-end race engine builder told
me that they took new engines apart, decked the blocks to get them more accurate, port matched the heads, milled the combustion chambers to like-volumes and
mixed rods to find 8 of the same centerline-to-centerline length. Until I started to build an L/78 engine, I never considered these issues.
I guess this starts to explain why some cars ran better than others ( having the same engine HP ratings ). Just thought I would share this information.