Buick 300 break in


Just finished rebuilding a 1964 Buick 300 .
I ran it on the stand to make sure there were no leaks .
I ran it three different times, but not more then 5min. each time
Now it is in the car and I am not sure of the proper break in procedure.
the ST 300 trans was also rebuilt , but it says in my book not to run the trans. fast .
How do I do this and yet break in the engine?
If you are running a stock camshaft, I would say, at this point, just take it out and drive it in a normal, comfortable, fashion. The biggest concerns about initial firing of a new engine are ensuring oil everywhere, you are past that. Normally you would prime the engine, maybe you did, but if you didn't the world won't end as long as you lubricated everything reasonably before starting it.

The biggest concern with initial break ins is cam/lifter failure due to heavy valve springs. You don't sound as if you have them, so enjoy the motor:)
Proper break in for a new cam is to fire it up and immediately bring the RPM to 2500. Run it at that RPM for 20 minutes straight. You can run it for 10 minutes, then stop to let it cool down, then run it for another 10 minutes if the engine gets too hot. 20 minutes at 2500 RPM is what you need to do. Doesn't sound like you did that.