Speaking from 4 years of Navy electrical/electronic experience, It's better to go big with ground wiring than with the smaller one. All your electrical return goes through one wire when going back to the battery. It needs to be able to hold at least 100 amps.
My suggestion is to run a 2/0 ground from battery to block, a heavy duty gorunding strap from the battery to chassis, and as a back-up another from block to chassis. That way if one breaks you have two back-ups and it doesn't get overloaded. If your battery to chassis ground strap breaks, the electricity attempts to find ground by any means necessary and will cause a feed-back and you'll start blowing fuses and can even have a fire.
Remember, your motor is isolated due to the motor mounts and if you lose the battery to chassis ground, you lose most of your negative return. The motor/chassis strap keeps the ground intact.
Another piece of advice is to connect that 2/0 battery wire to your starter. You'll have less voltage drop, hence, more starting amperage.
Hope this helps.