I cleaned up the replacement panel for the right side of the dash and marked and cut the slot, drilled the neighbouring hole, then file-fit it ready for welding.
I made and welded an insert piece to the right hand inside of the part, then slotted this inside the right of the gap to provide leverage and reinforce the fit. It worked well. I welded the lot in and ground it flush.
The radius, incidentally, is correct and was achieved using my shrinker and stretcher. The part looked like an oblong but there is a subtle arc.
It really is so much easier welding fresh metal. The weld for the insert hissed sweetly; it sputtered more when joining the replacement part to the old metal. I say new metal, but of course it is actually taken from the roof of a scrapped Ford Prefect with layers of paint and rustproofing removed. It is nice, thick clean stuff....from when cars were built to last.