I'm wondering the same thing..
I'm currently in the process of getting boost on top of my factory blocked stroker and was wondering what a safe boost level would be knowing my block is the weak link even though I have a main girdle. DSS swears by them, saying their girdle allows stock blocks to handle up to 650 HP reliably and that even extreme(1000HP) 4 bolt blocks will see benefit using their girdle. Knowing that steel and aluminum expand at different rates, to me, this in itself would cause main caps to become misaligned.. But that's just my opinion as I have no testing to back it up.
My previous factory block cracked down the middle, where the cam is. This was at mid range RPMs with ~15psi from a T-trim. I've seen where they now make galley girdles which are supposed to help in this area as well. I have not read anything good nor bad about them. They make sense in theory, but still wouldn't have helped my overboost at mid RPM situation as the crank does not have enough iron around the journals to prevent this..imagine a 400 lb person riding on a kiddie bike, putting all of his/her weight into the crank and the crank bearing cylinder breaking apart from the rest of the frame. You can add metal above the bearing cylinder, and you could add some below, but it still doesn't help the existing bearing cylinder(crank journal webbing) hold any more power.
On the other hand, if the high RPM, bearing cap walking is causing the failure, I would think the main journal girdle would provide some support, similar to how rocker stud girdles help the valve train.
Procharger uses 17psi as the magic number for their superchargers to double the NA power of an engine. So you have 300 HP now and put 12 psi on it you would use the formula (12/17psi)300+300= total FLYWHEEL HP which is just above 500HP. To get 600 FWHP from a 300 HP engine you would need the indicated 17psi. Again, the more you make the supercharger work, the less efficient it will be. IMO you should run a higher compression motor, 9-10:1 and run less boost.
Here is some good reading which may help guide you through your parts buying process.
MotorSports Digest
note the injector chart, your 30# injectors will suppport 450 FWHP at 40 PSI, to get your goal of 600HP your fuel system would need to support 80psi. So an injector/pump upgrade would be in order as well. A 255 with 42# would get your 600 HP, add a BAP to get your 700+.
I'm sticking with 8# or less on my stock block which will make just over 600 FWHP. I'll probably order it with the 8# pulley which would give me around 6# with my extra cubes. My 36# injectors with 255 in tank and BAP will be sufficient(close, but good enough) running around 55PSI and a 1:1 boost referenced regulator.
My personal opinion is that 600 HP is pushing it as long as you keep a SAFE tune..don't fall into the urge to run a bit more timing or a few more pounds. I agree with the posts above...500 on the street is hella fun...Ive been in 650HP vettes and it's really pushing the fun level back on the track...if even then.
-Mat