Used springs may or may not be OK. They can get weak from use.
Stock /6 spring rate is around 250 lbs/inch. Any spring with a wire diameter of around .19" will be the same, so you have to go up to about .20" wire diameter got get any stiffer spring with a single spring. And, you have to look at coil bind height for this cam since the lift is a good bit higher than stock (at least compared to the early /6's). The 340 springs do that but it is a dual spring. Sealed Power VS-678 may do it too, but check the OD to check fit. At $4 per spring, it is also a dual. (Not sure what you end up with if you pull out the inner.....)
BTW, the stock piston is a flat top.... so the CR will only be higher if the head or block was shaved.
