:welcome:
Hello mate - lovely looking 90! And I wish I could have had one as my first car!
And as for seats in the back - its a long contested argument. If you have a search you'll find endless conflicting explanations lol.
Check you V5, see if it states how many seats the vehicle has... You'll be limited to that less you tell VOSA or DVLA.
If you worried about legality, ring VOSA or the DVLA or something like that and I'm sure they'll tell you.
If you're not worried, grab some side facing bench seats, and chuck some lap belts in there, make sure you put spreader plates on everything though, don't wanna pull the seatbelt bolts through your aluminium tub! Wouldn't worry about a roll cage, but I'd get some sort of a roof, less prying eyes, and looks 'normal' you'll have far less questions from plod.
Oh and don't forget you'll have to tell insurance!