No, your 8" barreled shotgun has a shorter mag tube. These Dlask barrels are made for the reguler 4-shot tube. Any regular barrel will have the "barrel-band" too far ahead for your too short tube.
The standard mag tube is 4 shells, a standard 870 capacity is 4+1. Yes these barrels will fit the standard tubes. 12.5" barrel is flush with your tube cap. 14" barrel extend further or is flush with a +1 mag extension.