[STM69]

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My cam calls for a 135# seat / 330# open valve spring. But the spring upgrade on the heads are for a 120#seat / 325# open. Will this work or will I have to shop for other springs? The open pressure is fine but is the seat pressure too low? If so, what kind of problems am I going to experience? The car will most likely never see 6000RPM (if that matters).

[BlackoutSteve]

You could just shim your springs to increase the seat pressure. This will also increase open pressure by the same amount, but more pressure is better than less when talking valvesprings.

To see how much shimming is OK, you will need to know your max valve lift and how much lift your springs will accept before coil bind.
To calculate the amount of shimming required, your spring's rate in lbs/inch is needed.

Typically, a spring with 120lbs seat with a rate of say; 350lbs/inch will need a 0.040" shim to bring it up to ~135lbs. This will increase the open pressure by ~15lbs also (340lbs). You must check that the 0.040" shim will not cause the spring to bind at full lift and have at least 0.060" before going "solid".

[BillK]

S,
The only one that can truly answer this question is the manufacturer of your camshaft. There are a lot of variables involved when deciding which springs will work. There is a good chance it will be ok, but they should be the ones to tell you so. Also, make sure you are comparing apples to apples as far as the two springs go. What lift is the open pressure computed at for the springs on the heads ? Same with the springs your cam calls for. They need to be measured at the same lift dimension.