Phil,
De Tomaso didn't make a twin pod dash in right hand drive form. English cars had a single pod dash right from the start. You will either have to cut and rework your existing dash, or find a RHD single pod dash to take a mould from.
Your next problem is the steering rack. RHD racks are no longer available. The last RHD racks that came out of the De Tomaso factory where about £400. I suggest having a bespoke rack made by Titan
http://www.titan.uk.net/steering.php they cost approx £350
Johnny