8' or 6'5" box

I have ample clearance with a 382mbq and a newer GMC 6.6' (6'7", or 79.4" to be precise) bed.  I think it really depends on how tight you need (or are willing) to take turns and back up the trailer.  I suspect that most folks don't truly ever need to make turns at greater than 50-60 degrees between truck and trailer.  Besides, if you're on pavement, a 70 degrees backup is awfully hard on the tires and suspension (at least it LOOKS that way to me), but everyone's needs and comfort levels vary.

I pull my 382mbq with a 2021 GMC 6.6' bed and Anderson Ultimate hitch with the kingpin adapter sitting backwards (minimum offset). This configuration effectively moves the trailer pivot point about to about 1" behind the goose ball in the bed. The newer GMC's (2020+) position the goose ball about 2" behind the axle, so the combo gives me roughly 3" of extra clearance compared to a standard bed with goose ball location directly over the axle (like older GMCs and, I think, the other truck brands).

I initially wanted to go with GenY or the Reese Goosebox hitch to keep my bed clear when not towing, but I have some *really* tight turns to make into and up my steep/winding driveway and then it gets even tighter when backing into my parking spot near the house.  That drove me to go with the Anderson to give me maximum possible cab clearance and maximum turn capability.  In the end, it turns out that I probably would have be fine with the GenY or Goosebox hitches, but the extra clearance and turning angle does give me slightly more room for mistakes and little more margin if I'm not on level ground.

As laknox says above, you'll want to pay attention to hitch-to-bed rail clearance on the GenY (or Reese Goosebox) and account for how that clearance might be reduced when backing over uneven terrain. This is an dimensional/clearance aspect that I did not consider when i was looking at hitches, so I can't speak to how they might work on my truck.

From a turning clearance point of view, I think you'll likely be perfectly fine with the GenY and 6.5' bed, but it will require planning ahead in order to avoid super tight turns.  If you're looking at a newer GMC model, you'll get a little extra clearance with the slightly longer bed with the ball behind the axles, but you'll still need to plain ahead.

The longer bed gives a better ride and increases the GVW, I personally won't have a short bed, I haul too much dry wall & plywood.