I wrestled with that idea for some time with my mini-truck.
There are a number of things to consider.
Ventelation is a real problem back there both in hot and cold weather. Side windows will help, but unless the windows are right next to them it will be pretty uncomfortable. There's also the issue of the exhaust, which is more likely to leak into the bed area due to it not be air sealed or ventelated like the cab is. And it will significantly louder than the cab, since there is no sound deadening materal. Some of that could be fixed.
Then there's the safety factor. You'll want to add a roll bar, because if you roll, the rear will have no protection whatsoever and they'll get crushed. Also, the canopy is likely to come off, which could cause further injury. Also, in the even of crash and fire or going into water or some other situation where one needs to get out quickly, the escape options are fairly limited while in the bed.
The seat belts need to be secured and drilling a hole through the bed just won't cut it as the bed sheetmetal is too think, IMO. A few large washers or some steel plate, might help as it least spreads the load out over a wider area.
And then there's head room. You'd be surprised at how little head room there is with a standard canopy, once you install seats. A high roof canopy will help.
This is just my opinion, but I personally just would not do it from a safety point of view. But if you properly secure the seats and belts and add a roll bar, such as maybe a used one from an older 4runner, it will be better than it could be.
Good luck.
