I know you already checked ALL YOUR CLEARANCES ARE CORRECT!RIGHT? ok first make sure the ignition timeing , coolent levels are correct, all the hoses are conected ETC and a good oil filter is installed, add a can of G.M. E.O.S. to the oil(G.M.part #1052367 )
EOS - Engine Assembly Prelube
Specifically formulated as an engine assembly lubricant. E.O.S. provides outstanding protection against run-in wear and piston scuffing as well as run-in camshaft lobe and lifter scuffing resulting from insufficient lubrication.)
fill the engine with a good brand of MINERAL BASE OIL of about 10w-30 weight (VALVEOLINE, ETC. anything with a S/G or S/H rateing
next prime the oil pump while turning the engine over BY HAND SLOWLY untill oil runs out of all the push rod/rocker arms (if it doesn,t find out WHY BEFORE STARTING THE ENGINE) this is a good time to roughly adjust your valves if you have not yet done so. once everything checks out get the garden hose running if you might need it to cool the radiator and if you have one keep a fire extinguisher handy, check the float levels in the carb if your running one or the fuel pressure in the injector fuel rails if your cars injected and if fuel pressure is ok and fire it up with a timeing light on the motor, finish timeing it correctly and get it to at least 1500rpm-2000rpm and keep it at least that high for 15 minutes, if everything looks good take it for an around the block tour for an hour too 90 more minutes, bring it home, let it cool and change the oil filter(after 100 miles change the oil and oil filter again to mobile 1 synthetic in what ever weight will keep 25lbs of hot oil pressure at idle, after you have changed to mobile 1 synthectic and broken in the rings and cam your ready to run the crap out of it at the 100 mile mark, by that time everthing is lapped in BTW I always stick four of these magnets in the corners of the oil pan sump , you will be amazed at how much metalic dust they pick out of the oil and keep from getting to your bearings,(either one works fine) http://www.wondermagnet.com/dev/magnet2.html http://www.wondermagnet.com/dev/magnet42.html
synthetic oil has much greater film strength that it slows the lapping in of the lifters to cam lobes and rings to cylinder walls to a great extent, yes you can use synthetic oil to break in an engine but it will take longer and in some cases the rings don,t completely seal to the cylinders for about 500 miles while breaking in on mineral oil everythings lapped in by 100 miles or 3 hrs of running time. and yes constantly vary the engine speed durring break in but it must be in the 1500rpm-4000rpm window for proper oil flow/splash lubracation of the parts, btw this tool adds greatly to the oil flow to the cam lobes, http://www.compcams.com/catalog/335.html
and read this
Im NOT recommending you rev your new engine to 4000rpm, I Am saying its ok to let a new engine get to 4000rpm MAX for brief periods OCCASIONALLY going from gear to gear shifting under light load as a max rpm level as you accelerate going thru the gears in the car! only a total idiot would take an engine with (o) miles on it and go drag racing if the clearances are set up tight for long term use as a street driven car without at least some breakin time even if its just an hour cycling through the rpm ranges on a dyno. during the first hour of running time there is FAR MORE HEAT ON THE CONTACT POINTS AS THE RINGS AND LIFTERS LAP IN and those magnets I mentioned pick up lots of metalic dust, and that can get embeded into your bearings without those magnets. BASICALLY what your doing durring that first hour and to some extent the first hundred miles is letting all the high spots wear away and the metalic dust formed wash into the sump in the oil pan where its picked up by magnets if your smart or by the oil pump and hopefully trapped by the filter BEFORE IT GETS TO THE BEARINGS ETC.another reason they tell you to keep the engine rpms low durring breakin is to try to insure the oil filter bypass valve stays shut by keeping the oil pressure in the mid ranges to hopefully allow ALL the metallic dust to pass thru the filter,btw the main reason they tell you to keep the rpms at at least 1500rpm is because the cam is mostly lubracated by oil thrown from the spinning rods/crankshaft and below 1500rpm there is significantly less oil being thrown around in the lower block by the rotateing assembly and that the cam lobes on a flat tappet cam need to have that oil film on the lobes for the metal to lap in and work harden the mateing surfaces between the lifters and cam lobes. that tool I refered to causes high pressure oil to constantly spray on that lobe surface greatly increaseing the oil flow to the contact area and helping to prevent lobe failures, it also helpd spray extra oil on roller wheels on roller cams/lifters, look here at lifter #866 http://www.compcams.com/catalog/261.html
notice the oil groove! that is there to do the same job!
BTW if you read my post carefully it says,change the oil filter after 90 minutes too 2 hrs and change the oil and oil filter again at 100 miles, the oil filter will be partly clogged with metalic dust and E.O.S.the first time the engine cools thats why I said change the oil filter the first time the engine cools I also said to change the oil filter and the oil at the 100 mile point thats to remove any oil that has remaining traces of E.O.S. and remaining metalic dust from the engine before you add the synthetic oil.this whole process should take about 3-4 hours max, thats the total time that that E.O.S. will be in your engine!