Advice on buying junkyard engines?

When I was in the business, I gave a 90 day warranty on what I deemed a good used engine. My engines were run, compression test done, and oil pressure tested. The guaranty was for the engine the way I sold it to them. If you took it apart, the original guaranty was void, but, I would guaranty that any internal part out of specs would be replaced free of charge. You would be surprised (maybe not!) how many knuckleheads are out there that don't know what they're doing. No way I can guaranty anyone else's work. If someone wanted buy an engine that was going to be a swap from car to truck or vice versa, I would swap the pan, pickup, dipstick and tube, etc, for no charge except for the cost of new gaskets and fresh oil. This would keep the guaranty valid and give the customer and myself the satisfaction that the internals looked good before they went to the effort and expense of installing it. We worked with them for other swaps too (like 2bbl to 4bbl conversions, new water pump, new freeze plugs, and others) for a discounted labor rate and the cost of any parts to keep a warranty in effect if they wanted it. Some didn't care because they were going to freshen it anyway, but, most wanted a bolt-in engine with a guaranty. Buy from a reputable firm that has a good guaranty if you plan on just plugging it in. Some will even install them for a reasonable fee. Be up front as to what you want to do to the engine in case they have restrictions on keeping the guaranty in place. Many will use heat tabs to reveal if the customer overheated the engine. If so, the guaranty would be void. If you can buy one from a recently wrecked car that can be started, that would be your best bet. Plus you could see what the mileage is too just to verify what you've been told.