the 429's in thunderbirds, galaxies, torinos and even some station wagons are a good starting point. By the time you put an aftermarket intake and a modern cam in them, the weak points of the factory engines are gone. While a Cobra Jet designation engine would be cool, a run of the mill stock 429 still shares many of the same parts. I sam a pick up for sale on craigslist today with a 429 and a C-^ running for 1500.00 and it already had an aftermarket aluminum intake on it. You could find a running car and get much of what you need and then throw your existing engine into it and resell it for most of your invested money.
The major difference between a standard 429 and a cobra jet were the heads. But the standard heads with a port and polish job may actually be better for lower rpm torque and day to day driving. Cobra jet and super cobra jets used mechanical cams and have guide plates and adjustable rocker arms.
Super cobra jets had 4 bolt mains. forged pistons and a better oil pump.
For modern street performance most people use hydraulic cams so they don't have the hassle of valve adjustments and do you really need a 429 CID engine to turn over 6200 RPM's?
Also consider the 460's which are just a stroked version of the 429-desmogged and with good head work, they can be made into monsters and are externally identical to the 429's
Look for 1969 to 1973 vehicles. 4 bolt mains are nice, but not a necessity.