This is one of those, "ask 5 different people, for 5 different answers" scenarios.
Yes, you want to vary the RPM. You also want to prime the oiling system. A long flat head screwdriver shank chucked into a drill will not work. Use either an old distributor that has been modified for this (cam gear removed, etc) or a tool like the one sold by crane. Thing is, youve already stabbed your distributor.... and this is ESSENTIAL
It should go like this-
Engine fires
You instantly rev the motor to 2500RPM
ENSURE you have proper oil pressure (at least 30psi)
Within 2-3 minutes, adjust RPM to 2000
For the next 15-20 minutes make 2-300 RPM variations, never below 1800 RPMs, but as high as 3000.
Dont make these changes abruptly either. Remember the lifters actually spin in the lifterbore.
During this time youre ensuring your temp, and oil pressure stay in the green. Leave the radiator cap off at first so that you can add water as your engine purges any air bubbles trapped inside the water jacket of your engine.
The SBC (and virtually any pushrod v8) oils the camshaft by splashing oil from the crankshaft. The variation in RPMs ensure the splashing pattern varies also, keepibg all 16 lobes of the cam slippery.