The four cylinder chassis frame has many similarities to the six but is different in too many places to trust a six cylinder drawing. There are no four cylinder chassis drawings in any of the manuals I am aware of.
Greg or John at Kilmartins would be your best bet. Their chassis frame is spot on dimensionally to the original so they must have development drawings.
That said, if you are checking for alignemnt or distortion issues, the entire chassis should be able to be checked on a dead level floor or frame alignment table for sqare and truth by striking arcs, checking diagonal dimensions from an absolute centre line, etc.