Dart 340:
The size of the squirter affects how fast (in relation with the pump cam) the gas will be delivered into the carb. If you are testing with the same cam (as it appears that you are) and no other changes, a "031" squirter will deliver "more fuel faster" than a "025" squirter with the same pump cam. As 72 stated in your other post, your street 340 will like more rpms. A street 340 with its short stroke will not like a large squirter dumping allot of fuel at lower rpms.
What pump cam are you using?
Note: Holley pump cams from smallest cc to largest cc for 30cc pump (black, pink, white, red, green, orange and then the blue is the largest). The pump cams are fun to test with because you can feel the difference after swapping one out. For reference, if you start with a pink one and then install a blue one, you can definately feel the difference in the throttle at lower "cruising" speeds. The blue one might be too much for off idle in a mild sb, but for cruise testing you can feel the difference.