The Pilgrims trailed 3-0 at half-time against AFC Rushden and Diamonds and were down to 10 men following Jamie Gaukroger's red card for apparent foul and abusive language.
Bark and Crashley cut the deficit with quickfire strikes in the 70th and 71st minutes before Rushden re-asserted a two-goal lead, only for Crashley and Bark to pull off a fantastic result for the Pilgrims with further efforts in the 80th and 83rd minutes respectively.
Boston (4-1-4-1): Nick Ivanov (Jake Frestle 46); Jamie Gaukroger, Harry Cranfield, Daniel Fisher, Nathan Stevens (Lewis Bontoft 60); Marshall Young; Xander Crashley, Mido Turki, Will Jackson, Jenson Bark; Harrison Holland (Jack Bastow 64).