Before it even gets started, yes, it true your stock engine has very little valve spring pressure, and valve spring pressure is what kills new cams, but for the price of the break-in compared to a rebuild, it's will worth the trouble to break it in properly. Race engine are total different animal, and most procedures are geared towards them. Don't go by the shop manual either, it's 50+ years old and oils are nothing like they were when it was written.
Here is how I would do it:
1) Have a good carburetor that works and doesn't leak, full of fuel, and not brand new and untested
2) Have the coolant system full, no air pockets, new or cleaned radiator, heater core hooked up and open.
3) Have a new fuel system hooked up, fresh fuel, fresh filter. Use a jug of fuel if needed.
4) Have a fire extinguisher, garden hose, wet rag, and a few tools handy but not under foot where you trip over them
5) Have an oil pressure gauge that is accurate and mounted where you can see it at all times
6) Have an infrared hand held thermo temperature gun handy for checking temp of radiator and engine.
7) Have the engine oil system primed before trying to start the engine, DO NOT use starter to prime the oil system!
8) Have the timing set before trying to start engine, search this site for tips on setting time with out running the engine
9) Have Break-in oil in the engine, yes it cost more, but not as much as a rebuild will when the cam goes flat
10) Have a new or fully charged battery, new cables the correct size, and clean grounds
11) Have a full exhaust system installed, or quiet muffler, YOU MUST be able to hear the engine
12) Have the valves adjusted to cold spec's
13) Have some sort of tachometer installed
A) Start engine, if it doesn't fire right off, stop and check valve adjustment, fuel system, and timing. DO NOT continue to crank
B) Once it starts, set rpm to 1500 to 1800 rpm ASAP, DO NOT try and tune at this point, get rpm up NOW
C) Set timing as it is running by ear, advanced is better then retarded, set where it sound best,
D) Monitor cooling temp, oil pressure, and general engine condition, KEEP IT RUNNING
E) Use garden hose to cool radiator if needed, KEEP IT RUNNING, but keep it cool
F) After 10 to 15 minutes of continuos high rpm, slowly bring rpm down to an idle
G) Now is the time to set timing, carburetor, dwell
H) Let idle for a few minutes to cool the engine down before shutting down
Try not to shut it down once it starts, little leaks and what not can be fixed later, if it sounds OK, keep it going. Break-in is only for the cam and lifters, rings, and bearings will need some drive time to break them in so don't change oil yet. The only way the lifter and cam contact area gets oil is from the crank throwing it around, the higher rpm insures enough is being thrown at the cam.
Options:
Use the garden hose plumbed into the lower radiator hose, vary the amount of water flow to control the engine temperature, use a long upper hose to run the water away from work area. This works best if the radiator is questionable. The heater core helps pull heat out of the system, so have it open and blower running if possible. Use a thermometer in the upper hose outlet, keep in the 170 to 190 range.
