I just had one motor rebuilt and to keep a very long story short it is a gen 4 big block that was reassembled with used block, heads, pistons, and crank. New valves, cam, and rings. The builder recommends 20 hours of running not below 1200 rpm and not above 3200 rpm. How does this sound? I have talked to a number of people who have had their motors break in done on the dyno and is 6 hours. My feeling is that 20 hours is on the extreme conservative side. I don't want to do any damage and will do the 20 hours if needed, but I hope to get more professional opinions

I've never gone that long on a break in.... If it's a flat tappet camshaft, I agree about the 1200 rpm idea to keep oil splash on the camshaft. As a matter of fact, even with a roller cam I don't let my engines run under 1200 rpms unless I'm docking. Usually one full day of cruising (2-4 hours) with varying rpms through out the day.... Then back to the dock and change the oil and you're good to go. If you have a solid lifter camshaft, it's also a good time to re-lash the valves and also re-torque head bolts.

I'm sure others have their own opinions but this has worked for me for many years...

Modern engines really don't "break in". Bores are now done to a very specific finish and the rings are lapped. Exceptionally little wear occurs in what you might call a break-in period. The camshaft is "broken in" in 20 minutes, unless it's a roller in which case there's no break-in. Nothing else in the motor "breaks in" because there's no other metal-to-metal.

As far as head gaskets, most have a specific requirement. FelPro's is one thermal cycle, then a re-torque. Skipping it or doing it differently is inviting failure. There are way too many engine assembles out there who seem to know more than the factory does on these things- and the info never gets to the owner.

Like Sunkin said, if it was built right, nothing really needs to break in. Getting rings to seat and breaking in the cam used to take hours, now not needed. I think Panthers procedure makes sense.