Do a search for "Monroe Severe Service" under the suspension forum. First post made on them.
They give a better ride than the SensaTracs while giving better handling.
If you insist on Sensatracs, 5801/5955 are the correct 9C1 PNs and what they recomended before the Severe Service was introduced. 5906/5966 for Impala SS springs. Don't mix them. You won't get a cush ride...
The Severe Service shocks, 550012/550013 work equally well with any spring.