The rules are in place for a reason.
Try Speedway Motors,
www.speedwaymotors.com over in the racing and oval track parts section.
Like cheyenneshortstep said long pumps were 73 up on CK truck applications. So if you feel that will work try searching the 73-87 classifieds.
http://67-72chevytrucks.com/vboard/forumdisplay.php?f=5
If this is a "race only", I question carrying the weight of a power steering set up in the first place considering its factory listed as 33lbs over the stock manual box.