Hello Steve;
For a federal 12 pounder field howitzer, the most likely choice for you would be the M1841 12 pounder field howitzer as this model was, by far, the most produced model. The M1835 and the M1838 models were made in much lesser quantities. The M1857 light field gun, known as the Napoleon, was in production in the north and the south but likely not yet available.

For the confederate army, several of their foundries made some 12 pounder field howitzers usually based on the M1841 model. The production of 12 pounder light field guns was just coming on line and, I believe these were too late for the Battle of Pea Ridge.

I suggest the M1841 12 pounder field howitzer as representative of the actual battle equipment. The production of this model exceeded the combined production of all other 12 pounder field howitzers (not including the Napoleon gun)and it will be the most available to acquire. This is a great advantage for you, the availabilty to acquire.

Hope this helps
Ron Black