Hi Matt
Quite a difficult one is this.
Normally I would say that once you assembled all of the chassis as far as & including the seatback area, it would be safe to fully weld the chassis.
The rear frame & diff cage etc could be welded in later
I guess you'r main problem is the fact that you cannot assemble the transmission tunnel as yet.
Personally I would be worried that the chassis may distort if that area was not completed before fully welding
If you only need to move the chassis from the build table for a short period of time, provided it has been tacked well, it should not go out of shape.
Once you know the layout of the tranmission tunnel the chassis could be returned to the build board for final completion