single seat for double for kids -bad idea?

I have a 92 trident which has 3 point belts on rear seat/bed. There is only me and my two children and when they were both in full booster seats with head restraint they would both sit on rear bench or one in front and one in rear. Now the older one no longer sits in booster I put her in the front as seat has headrest. The younger boy will be out of booster soon and I am worried about him sitting in rear with head projecting above seat cushion as this seems perfect recipe for bad whiplash injury in even relatively minor collision. So.....was thinking of fitting double front seat but only ones with proper three point belt for middle seat don't fold and obviously won't swivel. Anybody done this or has an opinion? Will it ruin my vehicle as acamper and where will we sit when eating - we're only little but squeezing three on 3/4 rear seat is a squash. I want to keep van and I know I'm trying to get 'modern' safety out of old vehicle but just after some ideas.

In the days before cars had head restraints as standard you used to be able to buy clip-on ones. Occasionally see them on Bays with period fetish owners. But can you still get them does anyone know?

I remember my dad had these on his Hillman Avenger! But I have read they are dangerous and have now bought double seat. Have seen a swing up metal frame for 'buddy seat' on Ebay which is bit pricey but wouls also allow us to sit in rear.