engine break-in methods and reasons?

[PushnFords]

Ah, see, I was assuming a given new engine was liable to be totally different than the old one. I figure a tune that's perfect for one engine could conceivably run its replacement way lean and punch neat little holes in the pistons, thus the concern.

Well...true. But if the carb is tuned and running good on the old engine it will probably be ok to fire and go through the break in period on the new engine. It'll need retuned before you drive it much but at least it'll fire and run. I've seen guys try to break in flat tappet cams and the carb settings were so off it took forever to fire...lots and lots of cranking, backfires, and dying before they got it to run steady. Years ago I put a 351W in a '66 Fairlane at work with a Holley I'd also just rebuilt. Turns out I'd mixed up the metering block gaskets when I assembled the carb at 4am (been working since 8am) and it dumped gas down the intake and wouldn't run. Not something I want to do to a fresh engine and a big part of the reason I set them up like I do now.