boiler sizeing on counter-flow system

i'm going to replace one of the boilers in my two family house, the one that heats the top floor.  the connected load is 320 sq. ft.. the system is a counter-flow with  a two inch  main 20' long.  i have a gorton #2 on the main and its wrapped in 2". 

i want to use a burnham mst series because of their effiency ratings.  the boiler in there now is an arcoliner rated at 360 sq. ft. of steam which has heated the place fine except that it drinks oil.  thanks to dan's seminars and TLAOSH i've taken care of the fine details in the system and it runs well.  even heat and not one bit of noise.

so, 320sq.ft. of connected load X 1.33 pickup factor=425.6 sq. ft. of steam

my question is; will the mst396 (396 sq.ft.steam) be adequate or should i go to the mst513.  even with a 3" drop header i' concerned about putting too much steam into that 2" counter-flow main.  i've read about how touchy the counter-flow systems are to oversized replacement boilers.

what would you guys recommend?

thanks in advance for reading this