All that said, isn't the valvetrain on a BBC notoriously heavy? I think that would explain the use of higher pressure spings.If you are just going to race it then 300 plus seat is fine but if you plan on driving it you will find out that is a problem!!
Just for a setup to compare I run less than 200 seat on hydr. lifters to 7200 and no missing even with 20 pounds of boost pushing, trying to stop the valves from closing. Look at valve weight and total valve train weight to know the real answer as the spring pressure has to deal with the valve weight, lifter weight, pushrod and the cam grind/ramp will change this answer. You can just over kill it and set it at 300 plus but like I said, thats not a motor that will not have an issue if you street it.
You want as little spring pressure as you can because higher pressures eats HP, are harder to start by taking more starter/batt to turn them over and high pressure springs wear out faster. Bigger is not always better unless you really need it.