Q:  How much power can the stock block handle?
A:  There are several answers to this, do you want a combo you never have to worry about, a semi-reliable combo, or a time-bomb / glory machine?  For a reliable setup 450rwhp seems to be the limit, anything above that and you are asking for trouble.  For a glory pass the stock blocks on turbomustangs.com have seen 600rwhp.  You must also take into account RPM.  Most guys on turbomustangs.com keep their turbo combos below 6000rpm.  But if you are making 500rwhp and revving to 7000rpm it is a safe bet you will crack before the 6000rpm 550rwhp guy.

Q:  What happens when the block fail due to excessive power?
A:  Typically the block splits right down the middle starting at the threaded bolt holes in the main webs (where main bolts thread in) and work their way upwards towards the cam bearings and ultimately end in the lifter valley

Q:  Will a main stud girdle help keep the block from breaking?
A:  Ha, ha, no.  All it will is hold all the parts together in a tidy pile.  Some of the failures pictured below were equipped with stud girdles.

Q: Is it possible to have a cracked block and not know it? Like the motor still runs fine with no noises.
A: Yes, in some cases where the block has not cracked completely, the only thing you may notice is a drop in oil pressure (assuming you have an oil pressure gauge).

Q: Should I turbocharge my stock 93+ shortblock, ie: won't the hypereutectic pistons break?
You can absolutely turbocharge a hyper-piston engine.  In fact 2 of the combos below had hyper pistons and the pistons were fine when the block split in half.  It all comes down to the tune.  I also have a friend who sprayed a 150 shot on his 93 GT for many years and ran 11.0's all day and NEVER had a piston failure.

Q:  Can I crack a Dart, R302 or A4 block?  ie: How much power can they handle?
A:  None of the experienced racers on turbomustangs.com have seen any of the above blocks break.  The blocks in question have seen up to 1600hp and beyond.

Stock block carnage:  We are assuming the failures below were due to power alone.  Thus we are assuming that the tune was good, the car wasn't detonating or running lean. This is a fairly safe assumption because if these engines were detonating the pistons would likely be in many different pieces.  :)

"BarryD"
HCI on a stock shortblock (HYPEREUTECTIC PISTONS) 
Equipped with a DSS main stud girdle
18-20psi of boost (with the exception of one failure at 26-27psi)
10.30's @ 136-137mph in ~3400# car
approx 620rwhp at time of failure
lasted 75-100 track passes, 10,000 street miles with a MINIMUM of 530RWHP, and hundreds and hundreds of "street passes"
 

"Enginerd"
Stock shortblock (crank, rods, hyper pistons, cam)
Equipped with a Probe main stud girdle
Twisted Wedge heads, 1.7 RRs
GT40 intake (ported)
Twin T4's (60-1 hifi)
36lb. injectors (48 psi base fuel pressure) 255 lph in-tank pump with stock lines/rails
No FMU
EEC Tuner to set timing (22 deg. total at WOT)
3300 lb. car
10-12 psi boost
C4 with 3000 rpm stall and transbrake
10.30's to 10.50's at 125/128 mph
20,000 street miles/about 100 passes
  

Steve Gould (58Capri)
HCI, carbed 306, 5speed, 6400rpm
"125 shot" of nitrous
Approx 480rwhp
10.50's @ 128-131mph @ 2900#'s
Lasted a full season of racing, 80passes down the strip.  ZERO street miles.
     
  

"Hoss331"
My first one split about 2 years ago, and my second split about a year ago. My second one does not really count as it froze and split the deck and 3 cylinders. I never got to take it to the track, only run it on the street for about 6-7 months. It dynoed 420 rwhp and 412 rwtq with a s-trim at 6psi and 10.8 AFR. I dont remember the motor ever detonating it just went out with a big bang.
factory shortblock
home ported TWs w/ 1.6r
RPM intake
Lunati cam 51025
Vortech S-trim @ 6psi (3.6 pulley)
6:1 FMU with 30lb injectors 255lph
3300-3400 lbs

"Silver93notch"
6-71 Blower
25 psi
10:1 compression
6500 rpms max
T5
4.86's
Split in half