I finally got a reply from Moroso Tech Help. The C2600 3-way adjustable shocks are rated for springs with rates of 200-250 lbs.

My scales came last night so I'll see exactly what spring I need later today, but apparently it is possible to select the wrong spring since there are springs rates available under 200 and over 250.

Thanks to the Door Slammers chassis book and my hard-headed thirst for details, I won't be making that mistake!