Mick,
Not sure what model Heym you were looking at but there is another thing to consider regarding rechambering these particular doubles to a flanged cartridge.

Some Heym doubles have the barrel spacing at the chambers so close that there is little room to accommodate a larger case diameter and definately NO room to accommodate the flange diameter.
A friend once owned a model 88 sidelock in 375 H&H belted mag and the chambers were so close together that they only just cleared the Greener type barrel extension.
The barrels were so close together that looking at them from the top it was hard to believe that they were chambered for 375.
If the gun you're talking about is the same action as that one my mate had then it may be too narrow to rechamber to flanged.
Worth checking out.