Magnums are hyper.
Hyper is way way way way way stronger than cast. Way. We split the block on a stock Ford 5.0 before we hurt the hyper pistons. The stock 5.0 roller block is only good to about 500rwhp (with nitrous) before it splits down the lifter valley. The magnum block is a lot stronger.
When spraying over 125 hp, its always a really really good idea to keep a close eye on oil pressure and change the oil often, checking for bearing material or other bad omens...
I don't know about the hemis, but those 4.6 4V fords have issues out the yang with cracking ring lands, so if it's thin, stay far away.
The new DOHC 5.0s don't have the ring land problem, and even with powdered metal rods and hyper pistons there are guys running low 9s on the stock shortblock. That's intercooled boost rather than nitrous, but you get the idea.
I would not be scared to spray a lot on a stock Magnum shortblock . Like I said, I'd take it slow and sneak up on big shot, but I bet you could spray 200 or maybe more without worrying about the pistons, and it'd be a no brainer with a progressive controller.
Sooner or later, the cylinder pressure gets too high, or the stock ring gaps are too tight, or the stock rod bolts give out, but there's a lot of spray before that happens, and if you're using head bolts instead of studs, you will usually start blowing head gaskets before you do an insti-kill with those other issues. I would not use head studs on a stock shortblock I was going to spray a lot. Changing head gaskets sucks, but it really beats the crap out of the alternative.
Just remember, all that said, detonation under nitrous or boost kills hypers dead. Use good solenoids, keep the timing under control, make sure the fuel pressure is good. Use the right plugs and read em! Use good gas. Use a wideband. Don't get greedy.
And remember, some kits give you a really short hose from the nitrous solenoid to the plate. the nitrous is 1000 psi, the fuel is 7 psi. If you're spraying a lot, its usually a good idea to run a little longer nitrous hose. Huge lean spike when engaging the system can be real real bad.