What are some good driving practices when breaking in a newly rebuilt 312 yblock? My rebuilder will do a 20 minute break in procedure with a special break in oil. He told me to drive it for 500 miles at varying speeds and bring it in for an oil change and will cut open the oil filter to analyze. Any other procedures or driving recommendations?

Danny;  That sounds great for break in. This is how I do it, Before it is started Pull the Dist. I use a heavy duty drill Motor, with a shaft that will go into the Oil Pump. Run it for 5  or 6 minutes, to make sure you have oil at the Rockers. and a manual oil Gauge, To make sure you have pressure above 30 psi. Plus  a manual temp. Sounds like he has it down great.
Then after the break in is done. Leave the manual gauges in and drive at normal speeds. Keep a very close eye on the oil and temp gauges. If it starts to heat, Shut it off and if the oil pressure starts to drop shut it off also. I try to drive up to 1000 miles at normal speeds.Change oil and filter. And always use the best grade oil. I always use manual gauges, Because I do not trust the idiot gauges. Good luck with the break-in