I will be traveling to Poland in March
I really would suggest not coming then if at all possible. Come in May instead. March can be freezing cold with lots of snow or fairly cold and raining all the time, it's very unlikely to be nice weather but quite likely to still have slush and ice on the ground. May, however, is superb, most probably the best time of the year to come. Move heaven & earth if you have to, but come in May (or even late April).
The only clue I've had regarding place is a post card from family in Pisz, apparently sent sometime in the late 1940s.
Pisz was never part of 'Russian Poland'. Pisz was made up in 1945 (or perhaps 1946, I forget the exact year) because the Polish name for the town too closely reflected the fact that it had been Prussian / German for the best part of a thousand years (when given the chance to vote in 1920, the region overwhelmingly voted to join Germany and not Poland).
Can I focus on searching for Dabkowski's in these regions?
Sorry to rain on your parade, but looking for Dabkowskis in that region is like going to be like looking for an O'Sullivan in Ireland: lots of them, far too many of them for serious research.
Kielce, Kalisz, Lomza
All places well worth avoiding.
Lublin
A stormingly brilliant city which should be on any tour of Poland, the most under-rated city in Poland.
Piotrkow, Plock
Piotrkow is pretty mheh but Plock is worth a visit: it has a fairly charming old town square, two very decent brewpubs and the best place to eat is the restaurant in the charmingly named Hotel Petrochemia Plock, where, as the tourist bumf for the city points out "The Head Chef recommends the alcoholic beverages".
Radom
A dire toilet of a city.
Suwalki
Nice but very over-rated.
Warsaw
Unmissable.