RE: Bruce Lee
No idea about Bruce mate - but I do know that he was using cutting edge advice at the time, as mentioned above.
There are entire books written on core workouts... to be honest bodybuilders usually "neglect" their cores. Hauling big weights around the gym helps, but it's not the same as real core strength.
Maybe start with an ab-monster, followed by stomach pumps, then some assymetrics on the squat cage and bench press. Bloody dangerous exercises though, you're liable to bust a knee or pop a shoulder. On squat days, I start at just 20kg on a wobble board, and go up to 50kg doing a deadlift, and putting it behind my shoulders, all on the board (start by getting the weights in position off the board). Then I move onto the cage to go for heavy squats.