383 stroker replacement - bad mileage

OK, so I took the plunge and put in a GMPP crate 383E stroker motor a few months back. Was a pretty easy swap. Instaled short tube headers and a new High torque starter. Transfered over everything
else, including the entire Vortec FI system (as GM recommend.
Set everything as close as I could get it. It fired right up and seems to run OK, but.... after 3000 miles and 3 oil changes, the thing just doesn't seem to have a whole lot of pep, and the gas mileage is horrible. the old 350 that had 275k on the clock delivered a mix of city/hy of 14.5 (on regular). This pig can barely get 10 on premium, and when I hook up my 24; car enclosed hauler, I'm way down into single digits (6-7). This is unacceptable.
My thoughts are I might have a clogged cat or possibly leaking fuel injectors in the spider, but have no way to test them. No codes are popping up, and other than a very slight exhaust leak on one header, no ticks or bad noises.
I'm stumped and could use your help.
Dan
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Leaking injectors can be tested with a fuel pressure gage, see how fast the pressure drops with the key off, second, the cat be tested with a back pressure gage at the o'2 sensor port, the year here would help also.And which 383 is this?.

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snipped-for-privacy@capital.net says...
It's a 98 GMC Sierra 1/2 ton 4x4 automatic
Had it on a computer today, showed intermitent misfires in #3 & #4. Replaced the plugs that came with the engine with single electrode platinums (mechanics reccy). Re-gapped to .60. Still have the int misfire according to the computer, but can't see or hear it. Wires are new, check fine. Distributor new (inc cap and rotor).

GM Performance parts HT-383 E
Dan

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Just went through a intermittent mis fire on my '99 Yukon 5.7 95k miles. I had 2 problems, my distrib. was bad and my coil was arcing out the side also causing erratic misfires. Once I got The coil out I could see the arc marks by the frame of it. It idled smooth and I couldn't see/hear a problem either. This is my 2nd Chevy with a bum coil, my '89 S Blazer had one go bad also. Secondary ignition probs will not show a code other than the misfire ones in my shadetree experirance.
says...

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Is the exhaust leak before the O2 sensor?
If so, the exhaust leak is allowing outside air into the exhaust stream, and causing the sensor to go into a false lean reading.
Do you smell gasoline, when you check the oil. If so, it's a good sign that the lines on the spider are cracked, or the fuel pressure regulator is bad. Pull off the vacuum line to the fuel pressure regulator, and see if it drips raw gasoline.
I hope this helps.
RK

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