Thanks to all the help from Frank, Waldek, Adriaan, Gale and others, I now have all of Interarchy compiling and running under GPC. 270 files, 160,000 lines of code, 290 Mac Style object types, all compiling and working.
There are probably a few glitches left here or there, but I've tested out the bulk of the program and nothing remains broken (that was not before - it is bug for bug compatible ;-).
Thanks all! Peter.