There will be an upcoming 64 bits Delphi compiler. Embarcadero promised it.

Florian (the architect of FPC) showed a first "Hello world" program for Win64 in March 2006.
This was remarkable since GCC and the binutils don't even support this target at this time.
In fact, FPC used its Internal linker on Win32 and Win64 platforms, just like Delphi does.

Here are some points on how you could make your code ready to compile under FPC 64 bits, therefore (I hope) under future Delphi 64 bits compiler.