The parts catalog for your year model contains the threads and lengths for the bolts but it doesn't show where they go.
All cam cover mounting bolts (57-85) have 1/4“x20 threads with either 8 or 9 screws.
The parts books list the length screws/bolts HD sold for certain applications.
If you have the correct length/amount of screws but don't know which hole they go in, you can deduce the lengths needed.
And historically, some lengths have either been misprinted (as in motor mount bolts) or changed at the parts dept. from time to time.
Either way, the hole depths in the gearcase are deeper than the screw lengths in the parts book (some deeper than others).
There needs to be at least 1/8” or more of space between the end of the screw and the bottom of the hole.
So if you don't know the lengths, you can still deduce the lengths needed.
With the cam cover off the motor;

1. Gauge screw length by uniformity (using known lengths from the parts book): ¹⁾
   Hold the cam cover so that the outside is facing up and drops screws in all mounting holes.
   Check the screw depths on bottom (inward side) of the cover. They should all stick out of the cover app the same length.
   If not, move them around until the ends of the screws on the bottom are basically equal in length sticking out from the cover.
   Then record the lengths so you'll know where the screws go next time you have the cover.
2. Gauge screw length by measuring the depth of a light seat (for unknown lengths):
   Cut at least three pieces of threaded rod about 3-1/2“ long each (or cut the head off a long 1/4” screw/bolt).
   Screw a couple in the gearcase as “holders” for the cover.
   Install the cover gasket and cover over the rods and push lightly on the cover to take up any space to the gearcase.
   Screw another threaded rod through the bolt hole in the cover by hand until it “lightly seats in the bottom of the hole.
   Mark the point where it peeks out of the cover with some tape.
   Remove the rod, measure from your mark to the end of it and subtract 1/8”.
   That is the longest screw you want to put in THAT hole.
   Repeat for all questionable holes. Buy/use screws that are in that range.
   Screws/bolts are normalized in size so you may have to go shorter when buying them from stores.
   If you can't find the right size screw(s), you can buy longer ones and cut them to the size needed.
   Note: It's best to use normal screw threads for this instead of a straw or other. The hole bottom is always longer than the screw ending seat.
   Drill bits are chamfered and when the threads are cut, they end before the hole chamfer ends.
   Using objects other than a cut off rod or bolt can easily end with too long screw lengths and cracked cases or broken screws.