Once moving I like to run 41 two notches off of center. (Same as Mike May and Sonny when running 315)
340 and 41 have less heating surface than say 315. some of that is due to the fact that 340 and 41 have more bracing on the front tube sheet, and have a wider mud leg than 315. less bracing, and a thinner mud leg allows 315 to have a few more tubes, thus increasing her heating surface. 315s folio shows total heating surface as 1021 sqft.
I know this from looking at the interior boiler pictures in the 315 book and comparing them to the interior of 340 during her recent re-tubing.