As Uwe (Westcott) mentioned I bought a Fluidyne radiator for my “Sleeping Beauty” and I did the measurement.
The dimensions are: 610 mm wide, height 460 mm, 90 mm thick.
All 4 corners are chamfered approx. 25 mm / 45°.
As this radiator is layed back for cars with MkIII-frame, the coolant entrance is angled approx. 20° upwarts.
I looked at my MKIV and assume this radiator will fit in. The Fluidyne radiator is approx. 10mm wider than the original radiator (which is approx. 600mm x 480 mm), there will be some space left between the small front tubes, however it will be tight . The coolant entrance will come closer to the body/hood – but also feasible.
The “Serck” plate has been offered once by Acton Customs Enterprises or is still available in combination with a top cover to create a more periodic correct look (no need for MKIVs):
http://actoncobra.com/index.php?main_page=product_info&cPath=41&products_id=118 .