I've just had the leather front seat squabs on the old Rover recovered at enormous cost. ;-)
Apart from the splits in the leather (the main reason) there was some damage where the seat belt tongue fits into to the buckle. The buckle is bolted to the seat frame, and the opening to it actually below the level of the seat. So a tendency to hit the leather with the tongue when plugging it in.
The obvious way would be to raise the buckle slightly. To just above the seat edge. Perhaps 2" or so. I'm sure you could weld in a new bit - but I'd be worried about the heat damaging the plastic bits - as well as strength. And bolting on an extension would have the bolt head rubbing on the seat side.
Now there are thousand of buckles with stalks to be found used on Ebay. But all for specific cars. And all you get to go by is a pic. Not found anywhere that supplies new stalks on their own. And complete new belts are unlikely to match the ones in the rear - as well as costing more.
The further snag is the tongue on these belts (Britax?) is a sort of arrow head design. Nothing like modern ones which tend to be rectangular with a hole in the middle. So would need a matching buckle.
I'd guess there was a BL car of similar vintage that used the same make seatbelts, and has the same sort of buckle attached to the seat frame, but longer.
Other thought was to change the tongue to a modern one giving a much wider choice of buckles - but I'd guess this would involve unstitching one end of the belt to fit it - something else I'm not keen on doing.