Jay, if you are using the stock 351W heads this might be the problem. Both the valves and especially the exhaust ports are too small for a supercharged engine of this size.
This causes boost pressure to build up in the intake manifold, the air/fuel mixture gets heated due to the restricion what shows as more boost on the gauge, but actually there's very little boost entering the cylinders.
Enlarging the exhaust ports as much as possible will help. This is a D8OE head I prepared for supercharging:
As long as you're up to it you might as well polish the combustion chambers, this will reduce the compression ratio (meaning you can run higher boost) and sharp edges in the chambers will be removed to avoid hot spots that will pre-ignite the mixture:
Port matching the heads to the intake also won't hurt.
I did all this work in my garage with hand tools. It isn't complicated, it just takes some patience.
With bigger valves stock heads prepared this way should be capable of ~500hp.
Simon
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