GT-40p headed motor

[89 Cobra LX]

I recently posted here regarding a new motor that I purchased from Excessive Motorsports. The motor is an Explorer long-block with a Trick Flow stage 1 cam topped off with a Cobra intake, 24lb injectors, "stage 2" ported GT-40p heads and 1.6 roller rockers. I was extremely disappointed with the performance of the motor after a recent trip to the dyno. I made 260 (not at the wheels) peak HP at a measly 4500rpm. I have spent countless hours troubleshooting and investigating possible problems. The motor was advertised to make 325 HP with the stock rockers.

I have come to the conclusion that my problem is with the valve springs. Excessive Motorsport did not upgrade the valve springs. I'm running a high performance cam with stock Explorer valve springs. Trick Flow recommends springs with 120lbs pressure closed. The Explorer springs are probably coming in at around 90lbs pressure. I have spoken with Dennis at Central Coast Mustang and he said the Explorer valve springs are an immediate red flag (he said the Explorer barely revs to 4500rpm). My man here at HPMotorsport feels the same. I've gone through all accessories of the motor and it is bullet-proof. There is something that is not allowing me to make decent HP numbers and the valve springs seem to make the most sense.

It will be very costly for me to have the valve springs replaced at this point and was looking for some advice from some of you here. Would it make sense to change the valve springs? I'm a realistic person and don't expect to see a 65 HP gain from changing the valve springs, but does anyone have any idea what sort of power they could be costing me? Bottom line, this motor should not be making peak power at 4500rpm, it should be closer to 5400rpm.

Thanks -

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Eric - 89 Mustang LX
302 - Cobra Intake - GT40p heads - Naturally Aspirated