How could I lose 71 hp from one dynojet to another?

[69 Mach I]

When car was still stateside, the shop that built it tuned it on a dyno and got 351 rwhp at 6200 rpm. I was very happy with it as this correlates to 438 hp at the fly. Then to prep the car for the inspection here, I sent it to a guy whom everyone here says is an expert in mustangs. Well, long story short, he tried to undo all the work the shop in PA did because non of it was stock and "everyone prefers cars in an original state in Europe." This guy was a complete fool. So I took the car to a shop that has its own racing team based around 4.6L ford mustang engines for a proper tune.

My first dyno run was 280 rwhp and 373 torque. That HP correlates to 350 hp at the fly. This sucks because the car had 300 hp when it was stock (351W) and you cannot tell me I only picked up 50 hp by boring the motor .060 over, adding forged flat top pistons at 11:1, alum cylinder heads, new intake, long tube headers, MSD 6AL and Pro-Billet Distrib, a new holley 650 cfm 4bbl carb, and a Crower cam with the following grind.

ADV DUR: INT 297, EXH 308
LIFT: INT 538, EXH 534
DURATION AT .50 DEGREES: INT 236, EXH 242 @ 108 centerline

They are going to do another run today and I hope to pick up some power. Otherwise, I am going to be terribly bummed out.

If you look at my build, I easily should be getting over 400 hp at the fly from my engine. If there is power to find in my engine, I am sure this shop will find it. What could account for this big drop in power?