Yeah, I tried the vice grip thing first, until I started shearing teeth off my vice grips (whatever the shaft is made of is HARD) None of them were as tight as that first one though, so the vice grip thing probably would have worked fine on the rest. Do you know how to decide how many shims are needed when assembling a transmission from parts? Also, do you know if there is any wrong way to install an H gear as 5th? That is my ultimate goal, and I don't want to screw it up, or change the laws of physics

The shims are used to line up the gears with the pinion shaft. The best thing is to _not_ change the gears. If you want to change the gear ratios, the easiest thing to do is to measure the gears you are replacing with a micrometer and then add or subtract shims to give you the same combined number.