If you're competent you can take the stock engine out and remove the rods and pistons. Measure the bores. If they're in decent shape, give them a hone to break the glaze, clean it up. Measure the rod journals (don't remove the crank.) If they're all good you can get the good parts:
- a set of Manley forged pistons (e.g. SHM DIF3.552 standard bore size) for $649
- rings (e.g. SHM 7032-STD) $149
- rods - Manley H-beam rods (e.g. SHM 14042SHM-8) for $599
- rod bearings (e.g. SHM 8-1985A STD)
and slap your shortblock together. The cast crankshaft will be fine for 500HP as long as you keep the RPM sane and the forged reciprocating parts will be easily up to the task. An added benefit of using the OE crank is not having to worry about the flywheel since Cobra cranks had 8-bolt flanges.
This type of DIY solution is going to be cheaper than sourcing a used Terminator SB or buying a pre-built SB. But it also assumes (a) that you're competent and have the tools to build an engine and (b) that your existing engine doesn't require any machine work. Of course if you've got lots of miles, a ridge a the top of the bores, taper or out of round cylinders or journals etc then you're in for a lot more expense and time.
Once you've got a decent forged SB top it off with a set of good PI heads with stock or Stg 1 cams and a blower at 14-psi. Of course a blower is big-$$$ no matter how you swing it. And make sure you invest in the fuel system and air metering (MAF) too. This means a Cobra pump setup in the tank (at the very least), 60-lb injectors (42 might work) and a Lightning MAF plus a PCM tune to tie it all together.
500HP is non-trivial. You'll want to think about the clutch and trans at that sort of power if you have traction. What wheels and tires do you have? If you race at all you'll probably want to think about upgrading from the OE 28-spline axles out back and invest in a driveshaft loop. The rear
suspension will need help getting that sort of power to the ground and SFCs can help stiffen the chassis. What are your plans for the brakes? It's one thing to be able to get to 100MPH in 10 seconds but you have to stop the thing too...
There's no "cheap" 500HP. It's not just about the rods and pistons. There's a bunch of other stuff necessary to make it all work.