Do a search on the topic. Kenny set out the procedure for checking the clearance for the PTO. Basically get a few gaskets. You also need to get the studs instead of the bolts used for the cover plate. What follows is what Kenny said in the post. These are his words not mine.
PTO gaskets are sold in varying thickness. The combinaton of gaskets is how to obtain the clearance. What you have to do is take the inspection cover off the PTO. Bolt the PTO up with 1 gasket and all 6 stud nuts. Reach inside the PTO and put your finger on the gear that engages the transmission gears. Rock it back and forth, if it doesn't rock you need more gaskets or a thicker gasket. Input gear clearance should be between .0008 and .0012 inch. I've been doing them by feel for a long time without problems.
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Kenneth Engle
If you have a dial indicator then the procedure is in the -34 tm and it uses the backlash method.
TM 9-2320-361-34
Ensure transmission output gear is stabilized while measuring
backlash.
Shift into second gear (TM 9-2320-361-10).
Install new gasket (4) and PTO (5) on transmission (1) with six nuts (6). Tighten nuts (6)
15-20 lb-ft (20-27 N.m).
Set dial indicator on PTO (5) to measure PTO input gear (

backlash.
a. Check PTO backlash. Backlash should be 0.008-0.012 in. (0.203-0.305 mm).
b. If backlash is 0.008-0.012 in. (0.203-0.305 mm), perform steps 5 through 7.
NOTE
It maybe necessary to use more than one gasket to obtain
0.008-0.012 in. (0.203-0.305 mm) backlash. Each gasket will change
backlash approximately 0.006 in. (O.152 mm).
c. If backlash is less than 0.008 in. (0.203 mm), remove dial indicator and PTO (5) from
transmission (1) and continue with step 7 using two new gaskets (4) to obtain 0.008-0.012 in.
(0.203-0.305 mm) measurement.
d. If 0.008-0.012 in. (0.203-0.305 mm) backlash measurement cannot be obtained, replace (para. 8-2)
and repair (TM 9-2520-246-34-1) transmission (1).
Remove dial indicator from PTO (5).
Remove six nuts (6), PTO (5), and gasket (4) from transmission (1).
6. Apply sealing compound on gasket (4) and transmission (1) mating surfaces.
7. Install new gasket(s) (4) and PTO (5) on transmission (1) with six new lockwashers
Tighten nuts (6) 15-20 lb-ft (20-27 N.m).
(7) and nuts (6).
I still have yet to install mine. If I get a chance today I will try to post picks of the items needed. Also check the -20p and the -34p as they have great exploded diagrams which show what goes where.