Stop in for a cup of coffee

So say you bought a car that came from the factory with a big block and 4 speed. The thing is it has non of the above when you buy it. Would it be sacrilege to install a small block and automatic in it seeings a how that is all you had?
Think 69 Roadrunner 383/a833 car to a 360/a904.
Its your car, so as you wish. That said, it obviously wouldnt be worth as much but the odds of you finding the numbers matching items would be astronomically low and probably impossible. Being a 383 car, which was the base engine in 69 i believe, gotta double check it, its not a high dollar road runner anyway. Run what you got