Seats are held in by 4 bolts underneath the car, they bolt through the floorpan into the subframes.
Pretensioners pull the seatbelt buckle socket down in an accident to tighten the belt to hold you in tight. If you haven't got them now (you might not) then you don't need to worry about them just leave the connector.
I'm assuming you don't have side airbags either, think they only started doing them in the Ph2 onwards.