if you hace some muts take off the zinc and weld them on.
if not can you make some? do you have a 1/4 28 unf tap? Some 1/2" mild steel would probably do. what are you trying to fix?
I think the problem is down to the metrification of the UK. Pleny of Meric weldons available, but American threads are less common genrally. There must however be a fastner suppliers in the UK that have 1/4 UNF weld nuts
Daft question but aren't you able to change the bolts to metric.
I know you've probably already thought that but are you wanting the UNF so that you are in keeping with the vehicles time period.
as a classic car owner, I hate the idea of mixing fasteners types, not because of originality but the confusion of spanner sizes. Personally I would just weld standard nuts.
If you can't get hold of weld nuts then you could always use rivnuts.
These just pull into the metal and self clamp,
I use these at work as we can't weld nuts into a body under warranty as per Mercedes benz warranty guidelines.