[Sprocket]

If you want to use your block, use it and run it until it blows, if it blows. Personally, I would get another shortblock, but that's just me.

Here is what I would do given the chassis, usage, and intended performance goal parameters that you have provided.

400 factory shortblock (the closer to zero deck the better)
Box stock 72cc Edelbrock d-ports
Edelbrock RPM intake
800 cfm carb
SDPerformance Stump Puller II hydraulic roller cam
RARE 68-72 GTO-style Ram Air exhaust manifolds 2.5" collector
2.5" mandrel bent exhaust front to back
TH400
3.55:1 ring and pinion at a minimum

That should get you damn close to 450 flywheel hp without giving up much low-end torque.

If you just want to do a lot of street sweeping without worrying about power much beyond 5000 rpm, look for a hydraulic flat tappet cam in the mid 220's/low 230's range, e.g. the Lunati 307A3 (which is the old Ultradyne 280/288) or the Lunati 60903.