If your k-member has motor stands, you can use the conversion motor mounts if you choose to.
<quote> Can I run p/s? <end quote>
That's up to you.
<quote> What kind of chassis mods will I have to do? <end quote>
Once again, that's up to you. Your 93 Mustang is your strip car, right? What chassis mods did you do to it? Take into account the power (torque) your 521 is going to have compared to your 93 and decide how stiff of a chassis you're comfortable with. The stiffer the better- some guys don't even do sub frame connectors or reinforce the control arm boxes. Others do a 10 point cage welded to through floor subframe connectors.
Ultimately, it's your car and you can do what ever you want. If you want to leave the p/s, you may have to fabricate to make it work, but that's your option. Do some searching on the internet and see what people with similar combos are doing. Then, see if they have positive or negative comments about the given mods. Base your decision on your research.
Have a good day!