67 was the only year that the SM420 had an integral reverse light switch. Earlier years, if you ordered the option, had an externally mounted switch actuated by a tab attached to the shift lever. This arrangement was somewhat fragile and finicky to keep in adjustment.
http://www.oldchevytrucks.com/blog/i...-light-switch/
Fortunately, the cover on any year SM420 can be drilled and tapped for the later type switch. This gives much more reliable operation.
http://www.kinzers.com/don/landcruiser/sm420rev_switch/
http://www.kinzers.com/don/landcruis...420_sender.jpg
As for your clutch shudder, the most likely cause is oil getting onto the disc facings, so remove the bellhousing cover and look for oil on the flywheel or pressure plate. Another possible cause is a broken mount which allows the engine to move when under load.
Ray