I probably have  2 or 3 different instruction sheets for installing  a PTO on a 99. That included 2 or 3 for the 6 cylinder models and the same number for the GM models. Some may even part numbers on them. I have not looked at those in years. What type of hydraulic system does your GM have? Also remember some of those PTO's had a 1/3/4 inch splined output shaft instead of a 1-3/8 inch.