Minimal Vacuum at idle

[DuckMan]

I know this topic has been beat to death, but I'm at my wits end. I've checked for vacuum leaks, disconnected and plugged the supply line from the tree to isolate the engine, replaced the hoses on the underside of the intake (GT-40) and still have only 2 inches at idle. Of course the idle quality is terrible and the car is getting terrible mileage. what else can it be?
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The car is an 88 GT with a 351W and an S-trim. F303 cam, MAC shorties, Bassani X-pipe, Pro-M 75, 36lb injectors, GT-40X heads, and was built as a blower motor from the start. I just had a mishap on the Duck so I really need the Stang to run. I've pulled the codes and get an 11 on the KOEO, and a 66 on the KOER. I know this indicates the MAF is not working correctly, but Pro-M thinks I should try and find the leak first before sending it back.

Any help is greatly appreciated, I am new to the forum, and look to be active. TIA

joe