First off - WELCOME to Team Camaro -
IMHO I wouldn't go to a lot of effort to modify a 'stock numbers matching' block for performance use if you are worring about having this block around in the future.
There are lots of cheap 4-bolt blocks out there for use in mild to high peformance builds that wouldn't need the investment in parts and machine work to convert your block. Add to that the fact that you will have to go through the engine (cam bearings, mains and rods ) anyway after the machine mods are completed and you would be better off starting with a machined 4-bolt block or assembly.
If your intent is to get into the upper 500+ HP ranges eventually you would be much better off starting with a GM performance block or an aftermarket block made specifically for these applications. You would then be assured the unit would hold up to just about anything you are going to use it for. Fully preped aftermarket performance blocks can be had for prices not far above the amount you would have in preping an older unit to serve your needs.
As far as your question about being able to reuse your rotating assembly - we would have to know a lot more about the parts presently in the motor.
The crankshaft in your original engine 350/250hp 2-bl carb "HD" is most likely cast - so it's not a very stout unit to base a performance build on. Same would propably go for the other stock parts internal to this motor if they were all retained during it's last rebuild.
You would be much better off to look into GM or aftermarket HD/HP parts for the rotating assembly to be sure they would hold up to your performance modifications.
The same logic is going to apply to the cylinder heads and other parts on your motor. If they are mainly reminats of the original 'HD' coded engine, they will most likely have to be replaced by higher performance pieces to achieve your goals of a reliable base to perform future upgrades to.
Hope some of this helps;