Dyno pull

So me and my dad got the 383 duster on the Dyno yesterday and I was shocked by the numbers we got ...theses are the specs that "we have been told " about this motor

69 383
Ported and polished stock heads
Mild cam( durations unknown)
Edelbrock dual plane intake
850 Holley carb
Over the fender headers ...look like 1.25-1.5 primaries
727 manual valvebody 3200 stall

Now the third and final pull in 3rd resulted in 243hp@5500 rpm and 258ftlb torque @ 2800

Why are these numbers low ..I thought for sure the way this thing drives that it would easy be in the mid 300 range ...were my expectations to high ??