I have 5 point belts in my Spyder and used to take the kids for short round-the-block type rides.
They fit fine in there when they were about 4 and up, obviously one at a time. In a Vintage Spyder there is a chromoly tube frame with side protection bars, so I always thought they were safe enough. Much better than a booster seat and a 3 point seatbelt anyway.
I also had a Vanagon Westfalia which only had lap belts in the back seat. I purchased aftermarket belts that were 4 point and satisfied my ex-wife for kid safety without having to buy a new vehicle.
I have pictures of my kids in the Spyder I'll have to dig them up.