If you search the CC site you'll find coilover spring rates have been discussed a lot. Regardless of which way you go, you'll need to know the front weight, and perhaps the rear as well while you're at it - so find a scale.
As a starter I'd read the following thread - it has a few suggestions, including the experience of someone with a 460 (including air conditioning!) and his thoughts the 400# springs may be too heavy and he could have gone lighter.:
Coilovers and MII front end
QA1 has a good article on spring rates at
Street Performance & Racing Spring Rate Tech | Understanding Spring Rates | QA1 Suspension
QA1's recommendations for Mustang II front ends are as follows, based upon front end weight:
<1350 lbs.: 375 lbs/in.
1350 - 1525: 500 lbs/in.
1525-1700: 600 lbs/in.
Shorter suspension arms may call for higher rate springs, depending upon the geometry.
Your best bet may be to call technical support at Viking, QA1, Heidt, etc.