Tested software/driver: Virtual Box Guest additions from VBox Versión 6.1.18 r142142 (Qt5.15.2)
ReactOS version: BootCD 0.4.15-dev-2738-gbe65a85-x64-msvc-win-dbg
It should be like in XP64bits, but it splash a error of wrong architecture.
I tested the same .exe with Whistler XP 64 Professional (5.2.3790.1218) and it installs and it can be used (
no WOW64 needed apparently). It splashes confirmations for the drivers to be installed.
Debug log: Windbg - ReactOS x64 VBox.txt