I'm finally building my /6, and have a question on adjusting the valve timing. If an adjustment is needed, should I use an offset key on the crank sprocket, or drill out the cam gear hole and use an offset bushing? I think the offset crank key would be easier if it works.

Both methodes work fine, truth is that the "off-set bushing(s) in the top cam gear is a little easer to deal with just because the bottom crank gear is a light press-fit, as are the "keys" in the keyway.

You need a good puller, more time and "skill" to adjust at the bottom location. This is also true on the "trick" JP "multi-keyed" bottom gear timing set, that bottom gear does not just slip on & off but requires some force to install & remove. ( I heat-up the lower gear to install it with a correct "draw-on" - screw type tool, not with a hammer-Please) Your standard "3-jaw" puller may not have enough room to get behind the bottom gear to remove it.
DD