Using a new engine ensures peace of mind during the swap process. Nothing is worse than finishing your project only to find that the salvage engine you bought is no good. New complete 3800 "test engines" come up for sale fairly often. These are engines that have been pulled from the production line for quality control testing or from crash test vehicles.
A new Series II Supercharged
A new Series I Supercharged
Used engines are also a good option if care is taken to ensure that you are getting a good engine. The initial purchase price is lower, and you may also be able to get the wiring harness, ECM, and lots of other needed accessories with it, which could save you a lot of money in the end. Most of the conversions that I do are using used engines.
A pair of used Series II Supercharged
