"For using a single centered chain, first run short chain hooked to both, and the long chain hooked to the center of it. I am thinking that would work. What do you think? "
Sure can do it like that! It'll put a bit of a side load on the hooks, but as long as they're attached (to the bucket) securely...just can't see a problem.
You can also use just one chain (with a hook on the end) by running it through both bucket hooks with a 2 feet or thereabouts extending out, then rigging the hook to the chain to make a "V". Think of the shape of a wire coat hanger with the wide end looped through your hooks. The longer the "V", the less stress on your hooks.
Something else I do occasionally is tie an old tire to the loader bracket when I'm lifting. The tire prevents damage to the grill if it moves if I'm carrying something rigged to the bucket hooks. This would be equivalent to putting a tire (NOT mounted on a wheel) between the bumpers of two vehicles if you need to give someone a push. Just acts as a bumper.
Frankly, I don't use the tractor to drag or pull something I can lift manually. It's got to be something fairly heavy.
I will add one thing...if you're going to use the bucket, hooks or a combination thereof, make sure you up the pressure inyour front tires!! I moved an old boiler (definitely a heavy lift for my little 670) rigged to the bucket using the lip of the bucket to carry it and the hooks to affix a rope that tied the boiler to the bucket.
I was wondering why the steering got so hard...well, the tire sidewalls were wayyy down! Didn't have enough air pressure. Darn near rolled the tires off the rims! Don't make that mistake!