If I were to properly install both seats and seatbelts into the bed of my truck would people legally be allowed to sit in the back? All I have been able to find is stuff about child seats and always wear a seatbelt, cheers

It would be a nightmare to do because the regulations are VERY stringent. The law regarding legal seating capacity is quite specific. You will have to follow several laws including Canada's Motor Vehicle Safety Act and its regulations, especially section 2.2 "Seating Capacity" of its Safety Regulation , and the Motor Vehicle Restraint Systems and Booster Seats Safety Regulations. Plus, you will have to follow Transport Canada's seat standards which you can read here: Standard Seat Assembly Specifications for Motor Vehicle Restraint Systems and Booster Seats Safety Regulations Compliance Testing.

That said, I don't know of any Canadian insurer who will provide regular coverage to a non-manufacturer who attempts to modify their vehicle that way. The risks are simply too high.