The best way to do this is to pre
oil the engine. Remove the distributor, use an
oil pump priming shaft on a drill motor, and pump
oil through the engine. Rotate the crank a few times so all the lifters get pressurized.
Even if you don't do this, the lifters should pump themselves up a few seconds after the engine is running. Set the clearance at just past zero lash, let them pump up, then adjust them down to 1/2 - 1 turn past zero lash.
You're breaking in a new cam, so the engine can't be idling while you're fooling with this. Get the RPM's up around 2000 for 20 minutes.