Depending on how much play there is in your +/-, I come up with 450-490lbs rate.
Hyperco's online calculator gives the same values as my math above.
Ridetech recommends a 475lb spring with your numbers.
Eaton recommends a 367lb spring (13-3/4" long) for a 50s truck or a 383lb (14-15/16" long) for "heavy engine".
Honestly, calling Speedway like Tuts 59 suggested is not a bad idea. They can give you some feedback and you can see if they will do exchanges if the one you order isn't quite right.
Bear in mind that a heavier spring will make the front end stiffer. So you might be better off with a longer, lighter spring. The length will make it ride the correct height, but the lighter will soften the ride a bit.
Running 15" rims with 28"-ish tires will also soften things a bit from sidewall flex.
