The tubes are 3". The 84-87 G body 8.5 is 2.75" (same as the 7.5). The G body 8.5 will bend the housing before the Mustang 8.8.In certain areas I agree with this. For a normal street cruiser, weekend track car I would hunt for one of these housings. But for something over 600whp the Mustang 8.8 housing are not tough. The tubes are nothing like what came in the Exploders. And C-clip eliminators for the Mustang housings are not reliable. But again, 2 different applications.
I spent many years building Fox bodies including working for a well known shop here in New Jersey. We had 8.8 rears in the 7's. With the typical spool, axle and girdle upgrades they are just as durable as any of the GM integral rears. You should be skipping all of the production housings and go right to a fabricated 9" with brace if you are making that much power that you are worried about bending a housing.