Will,
We find that the "Impact Shield" type child seats work well with the standard lap & diagonal rear seat belts in our 1985 4 seater .
Our seats are a Cybex Gold and a Kiddy Phoenixfix Pro 2.

Jack
1985 4/4 T/C 4 seater Royal Ivory
1934 F Type
2018 Pembleton SuperSport (self built).