my opinion is number one, since on the dyno you can load up the engine as much as you want (and rev it as high or low as you want at that load). i know people who have done this and had no trouble. some people go to the trouble of putting a lot of weight on their car (towing etc) so they can increase the load to bed rings in - but a full load run through 2nd gear even to 4000rpm is probably hard to do, especially without speeding.
however this is just my opinion and i've never rebuilt an engine myself.