Take a lookk at this. Mav 8 and Mossberg 500 barrels appear to be interchangeable if the barrel is made for your magaizne tube. http://www.maverickarms.com/pages/barrels.htm
A Mav 88 barrel designed for a 6 shot would fit on a Mossberg 500 with a 6 shot but not a 8 shot magazine. I do not know if you can put an 8 shot Mossbeg magazine on a Maverick 88. For some reason I do not think you can add a mag extension to a 500 and I that may carry over to the Mav. I would check with Maverick/Mossberg to see if you can install the 8 shot magazine on a Mav.
If it was me I would just get the barrel for $81.58 and leave the magazine alone.