Selecting the right 1965 Mustang 289 engine for your build isn’t just about finding any small block—it’s about matching period-correct specs, condition, and your end goals. First, always check the block’s casting numbers; genuine 1965 units typically feature numbers like "C5AE-6015-E" on the passenger-side block rail and may include date codes stamped near the starter pad. For concours or high-value restoration projects, I recommend a close examination of head and intake manifold casting codes as well. Original K-Code HiPo engines bring significant value but are understandably rare and command a premium. Most street-driven restoration builds will do well with a standard 2V or 4V 289 with excellent compression and minimal bore wear. Grab a micrometer and check cylinder diameters—anything over 0.030” overbore means you’ll want to inspect the cylinder walls for potential issues. Swapping in a rebuilt or used 289? Don’t forget details like correct water pump rotation (early Mustangs use passenger-side inlets), canister-style oil filters, and the classic Autolite 2100 carburetor for a stock look. In my experience, verifying included accessories—such as factory-correct exhaust manifolds, timing covers, and engine mount bosses—can save you days of searching for correct-fit replacements. If you’re planning to run modern fuels, consider hardened exhaust seats. Finally, ask the seller about compression test results, oil pressure readings, and any modifications. A bit of up-front diligence ensures you get an engine that lives up to the Mustang legend from the very first spin of the key.