Boiler Sizing

Forgive me but I have another boiler sizing question.



I have a 15 year old Dunkirk boiler (model # 4ES1.85C) which is 259MBH input, 215 MBH Heating Capacity and 671 sq ft of radiation.



I have a small leak in the above ground oil tank and am doing a gas conversion. I was initially considering simply installing a Carlin EZ - Gas, but the install prices are so high I am thinking about changing the whole boiler (esp. considering the utility will give an $1000 rebate for the boiler swapout).



So I the plumber I was thinking of using is recommending a Burnham IN6 (175MBI input, 144 Heating Capacity and 450 sq ft of steam. At first I think this boiler must be too small (based on the boiler I currently have). So I tried computing the sizing myself...



In my whole house I have 3 radiator types.:

-8-3/4" tube type is most prevalent (68 sections in total split across 7 radiators)

-5" tube type in baths (10 section split across 2 radiators)

-20" tall sunrad convectors (28 sections split across 2 convectors)



If my calculations are right, the big rads are 238sqft of steam, small rads are 23sqft and the convectors are 63sqft. So I have a total of 324sqft.



So while I originally thought the IN6 was undersized (based on what I have in the basement now), I am wondering if its may actually be oversized. Thoughts?