Diagnosing vibrations can be maddening, but the key to pinpointing them is to eliminate as many variables as possible without replacing a lot of components that might have been perfectly acceptable. You seem to have done a good job of narrowing it down a bit. Since you replaced most of the steering linkage, we'll assume the stuff you had before was worn out anyway, and hopefully the shop that did the alignment is proficient (many aren't, particularly on older cars). Problems with frontend wear or bad alignments usually result in a bad steering feel, like pulling to one side, and of course, excessive/uneven tire wear, but it usually takes a severe problem in this area to create a shimmy, which you would probably feel distinctly through the steering wheel.
Your problem initially sounded like a driveline vibration, potentially the result of an out-of-balance driveshaft or incorrect universal-joint working angles, but then you mentioned that it's related to rpm, not necessarily speed. If this is the case, it probably isn't a driveline problem but may be an engine balance issue. As another test, take your car back up to the speed that induces shimmy and put the trans in Neutral to coast. If the vibration disappears, the driveline is probably OK (though U-joint issues are sometimes only apparent under load). If you can induce the vibration by revving the engine to about 2,500 rpm in Neutral, that's likely where the problem lies.
This doesn't necessarily mean that the problem is inside the engine, and in fact, it's more likely external. You seem to imply this condition didn't always exist with your Nova, indicating that something has changed on its own, rather than the shake being the result of something done during the engine swap. First, check the harmonic balancer at the front of the crank. Stock units are made up of two metal rings bonded together with rubber. It's not uncommon for old balancers to come apart, where the outer ring "walks" off the inner hub; this could easily cause an engine shake. If this is the case, it will probably be obvious when you look at the balancer with the engine running, though sometimes the outer ring only rotates slightly, which is harder to spot. With the engine off, inspect the balancer to see if the rubber is squeezing out between the two rings.
Check some of the more minor external rotating parts of the engine as well, like the fan and its clutch if it has one. We've felt engines that seemed as if they were about to come apart, and the culprit was a bent/broken fan blade. Another time it was a burned-out fan clutch that allowed the fan to move off center.
If the vibration only happens at that rpm while in gear, it could be the pilot bushing or bearing (your car would probably have had a bushing, but could have been changed to a bearing later) in the back of the crankshaft that supports the gearbox's input shaft. If this were the case, you'd probably feel the vibration in the shifter handle. The pilot bushing/bearing should be replaced whenever the clutch is changed, but lots of guys (and some mechanics) skip this step since removing the old piece is usually a chore.
More Nova TechI have a question that I hope you guys can help me with: I am restoring a '73 Nova on a 20-year-old's limited budget, and I'm currently putting the front subframe back on the car. The front body mounts were completely shot, and I have already cut off the tops of the mounts so I can weld in new pieces. I would like to know if there is a template I can get to tell me where to drill the mounting holes in respect to the framerails.Nick TripodiVia e-mail