I had one 350SDL, pulled apart a perfect-running engine and found that there was significant piston scuffing. The geometry of the 3.5L seems to be a problem, search "rod bender" to find out more, possibly a bent rod or breached head gasket (which could lead to a bent rod).

As far as power, search "switchover valve" and "overboost valve". There must be a clear path from the small vacuum/pressure valve on the top of the manifold to the ALDA on the IP for the car to get boost enrichment, without it you'll have much less power. Run the line from the manifold directly to the ALDA bypassing the switchover valve as a test, often the switchover valve gets plugged (clean with WD-40 or whatever) and sometimes the fitting on the manifold gets plugged (blow through, both should be clear).

Next be sure that the wastegate is hooked up, I've seen two with the linkage loose from the actuator.

Shift points are important, if the car can't get RPM it won't develop peak power.

Fuel and air filters. Without fuel, it will not develop peak power, without ample air it also won't develop peak power.

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Gone to the dark side

- Jeff