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Tracklocross Race Machines of Awesomeness

A tracklocross bike can start life as any number of other types, but in all cases except a track bike, the conversion is mostly a lot of subtraction. The best candidates, other than a purpose-built frame by Squid or Surly or Destroy, are 1970s road bike frames because they are rich in the following attributes:

horizontal droputs - essential for chain tension
~120mm rear dropout spacing - so track/singlespeed hubs will fit (126mm will work with steel frames, as they can be bent slightly.) No you don't have to use a track hub (Squids use 135mm spacing), but they are plentiful and used track wheels are often cheap.
tire clearance - most 1970s road frames have plenty of room for 32-40mm gravel/cyclocross tires.
standard 68mm British bottom bracket shell - easy to get square-taper bottom brackets such as a Shimano UN-55 (103 or 107mm width) that will work with most track cranks.
What else are you gonna do with a 1974 Schwinn Super Le Tour? You could build it up as a period-correct road bike I suppose, but other than look really cool, it won't do anything as well as a modern road bike. But as a tracklocross rig it's just as competitive as anything on the "circuit" (ha), and you'll get all the hipster cred.
A note about track frames - most have very little tire clearance so they're not the best for tracklocross conversion. Some do, like the Traitor PBR bike below, but they are uncommon. And you'll probably need a different fork.