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Questions:
Do the figures sound accurate?
Would the volume of that particular blower be too small, requiring a ridiculously high blower RPM to even boost 435 CI?
Would it be ok at 4 PSI without an intercooler?
Would the heads be less sensitive to porting work because of the boost?
Would the low RPM torque curve be much fatter than a normally aspirated 435" 500 HP engine and at about what RPM should the torque start coming on strong?
Joe's answer on power is roughly correct, although the blower takes power to run too, and that will subtract from the gains. On my 489" supercharged engine, at 17 psi of boost the blower took over 100 HP to run.
I think that blower is too small to work on your setup. You have to start thinking of horsepower in terms of air volume per unit of time. The more air in terms of cfm that you can put through the engine, the more horsepower you will make, blower or not. The blower in the ebay ad may not provide the airflow to even get you back to your existing, unblown horsepower level. In that case it will make zero boost and potentially cost horsepower. If you could find some data on the blower you could figure that out for sure.
I don't think that an intercooler is ever really required, it will just add horsepower if you use one. The more boost you run the more the intake charge is heated, and this leads to less combustion efficiency. Having said that, running higher boost levels (and heat levels in the intake charge) can lead to detonation if you are not careful, so limiting the boost to 7 psi will minimize this problem. You can also look into a Boost Cooler setup, which sprays a methanol/water mix into the intake tract before the fuel, to cool down the intake charge.
Anything you do in terms of porting, big valves, big cam, etc. will give you a directly proportional benefit with supercharging. You can look at this one of two ways; build your engine with standard ports and valves and a small cam, and use the boost to increase your horsepower by some percentage, and you have subtracted the cost of the normal performance tricks from the build. Or, you can build your engine with ported heads, big valves, and a big cam, and use the blower to increase that power by the same percentage. In other words, if you build a 400 HP engine and add 8 psi of boost, you will have a 600 HP engine. If you build a 500 HP engine and add 8 psi of boost, you will have a 750 horsepower engine.
Boost will always fatten the torque curve, but using a supercharger it will not come on immediately; it will be engine speed dependent. So, at 1500 RPM you may not get a big boost in torque over a naturally aspirated engine. At 4000 RPM you certainly will.
For what it's worth, I was very happy with the Vortech centrifugal blower that was installed on my Mach 1, and I was able to get over 1000 HP out of the 489" engine, running 17 psi of boost. It would be pretty easy, to get 700 out of a 390 stroker engine using the same setup and a reduced boost number. You will be spending a pretty fair chunk of change setting up your engine with a blower, so why not go for a little more power? Once you've got the blower adding boost is just changing pulley ratios. And if there is too much power, you can always throttle back
