Engine out of a junker

the engine in my car is out of a junker. the junkyard that the engine came from said that the engine had about 53000 miles, or so, and they did not know how long the engine had been sitting. how long should i let the engine be broken in before i can begin to drive it a little bit harder? i just put a brand new tranny mount in it. Once in a while, when i downshift into 2nd, it still bumps hard. consequence of the tranny mount being brand new and not being used/broken in yet?

if you pulled the engine from a car, that had already been driving it, no break in is required. the only time you have to break in an engine is if you put a new cam, pistons, or other rotating assembly parts into it. i usually say do the 15min break in, and then drive it like a little old lady (from somewhere other than pasadina) for around 500miles at least. trannies dont need break in time, and rear ends i would give the same courtisy of 500 miles. thats just my rule of thumb that has served me well. dunno if there is a specific break point that is in a book somewhere.

k. since i bought the car, i've put about 700 miles on it, so i should be good? i don't roast too much with it. i just want to make sure that the engine should have had enough time to get used to being used, again. when this engine eventually does go, hopefully not for like another 60000 miles, i am going to drop in the ls1, if it's not too expensive.