One problem with Nailheads is the left-side starter location, which causes interference problems between the steering, starter, and exhaust all trying to occupy the same real estate. Unfortunately, we know of no one then or now who made an adapter plate (as they did for Olds V-8s) that swapped the starter over to the right. If you have an M2 frontend, there's plenty of room to run a D-shaft past the exhaust and starter, but you'll have to plan carefully to utilize a Nailhead with a dropped axle, split 'bones, and a Vega box.
Your search for a Nailhead to use in your next project should probably be focused on the 364-401-425 engines. The earlier engines are harder to find, and the later engines have bigger valves and other improvements, although vintage speed equipment and trans adapters for the earlier engines are sometimes easier to find, perhaps because hot rodders "traded up" when the bigger Nailheads came out. The 401 happens to be one of the most common because it was produced in higher quantities over a number of years, while the 425s are tougher to find and are more expensive, since they were only in a few models. The Riviera Gran Sport enthusiasts are always trolling for 425s and pay a little more than we vintage-rod builders.
As with any senior-citizen engine, you should check the condition of the block and internals carefully, looking for cracks in heads, cylinders and water jackets, gouged cylinders from OEM wrist pins that came loose, or a loose balancer bolt (they need to have 225 ft-lb of torque to stay on) that could have caused damage to the crank snout. Take the short-block to a machine shop for a thorough exam. Ideally, find an engine that has not been rebuilt before. The Buick cylinder heads are comparatively easy on valve seats, but the material around the seats isn't copious, so many a Buick head has been ruined by someone trying to install hard seats and hitting the water jackets.
You'll find engine rebuild kits readily available from Egge Machine, Kanter Auto, and Centerville Auto Repair, with most internals still available from Federal-Mogul and oil pumps from Melling. Other Buick parts specialists include Classic & Muscle Automotive, and J&C's Parts. Between these experts, you can set up your Buick with everything from modern crank seals to roller rocker arms. Poston Enterprises has a line of performance camshafts for the Nailheads. You should know that the original profiles on stock Nailhead cams were comparatively radical for non-musclecar OEM applications, as a means of compensating for shortcomings in port size and angles. Most of the common Nailhead-building mistakes are well covered in the previously mentioned '97 Frohmader articles in Street Rodder. What is needed is a decent book about the Nailheads, because the story of these engines today is too big, even for an entire issue of a magazine.
The end of the Nailhead era saw these engines fitted with GM's newest transmission, the Turbo-Hydramatic 400, and we bless those forward-thinking GM engineers, as this is a rare case of a vintage engine that comes with a thoroughly rugged, modern transmission! These were in '64-66 models, but then if you have a pre-1964 Nailhead, that '64-66 TH400 trans and flexplate can easily be fitted to the early (shorter) rear crank flange with an adapter from Centerville Auto Repair. Wilcap has adapters for GM automatics and a stick bellhousing.