according to the corvette black book (bible) by mike antonick, the 1981 corvette VIN sequence went from 1G1AY8764BS400001 through 1G1AY8764BS431611 for st. louis cars. bowling green cars were 1G1AY8764B5100001 through 1G1AY8764B5108995.
1981 was the first year to use a 'G' as the second digit, so unless that part of your VIN is wrong, your car cannot be earlier than 1981.
i think the second digit (letter) actually is a 'G' rather than a 5. at this point everything is pretty much falling in line that you have an '81. i'm still unsure how you have an 'X' in that 9th position. the book says it's a security code and varies but i thought they only used numbers, not letters. but hey, i've been wrong before.
the engine codes seem to back everything up also and that appears to be a factory L-81/auto car.
so now, what you have is a 1981 corvette built at the bowling green assembly plant with the L-81 engine and the turbo 350 automatic transmission.
a few things you can check to further confirm this would be to take a look at the rear differential. it should be a big aluminum job that has "batwings" supporting it from one side of the framerail to the other. with the automatic trans, you will also have a fiberglass leafspring in the rear. in the interior, your going to have that idiot 85mph speedometer (federally mandated) and with an automatic car, just above the radio on the left side of the gauge panel will be a clock that doesn't work. if you fold the driver seat forward and open up the battery compartment and look in toward the front of the car, you may even see what's left of the computer. in the engine compartment, you should have magnesium valve covers and a MAP sensor bolted to the firewall on the driver side next to the brake booster.
what does all that mean? well, nothing really, but you know more about 1981 corvettes. don't you?
and what do you need to do to fix your immediate problem? first, yank that damn single plane manifold off and replace it with a good dual plane. then you definately need to either have that holley rejetted and tuned or replaced with a smaller job.
the fact that you stated you have black soot coming out of the tail pipes not only indicates that you are running waaaay too rich but in all likelyhood, the cat converter has been damaged by now. if you don't have to have it there, pull it off and replace it with straight pipe for the time being. if you are supposed to have it, pull it out and punch through it to get the crap out of it. i promise that if you are worried about smog (much less gas consumption), you replace that intake and carb and your going to run cleaner and get better gas mileage without that cat than you would with that cat and being way over carburated.
BTW, a 1981 corvette running properly will get about 20mpg (US) so your way off.
oh, and as for ecklers, they have probably dropped alot of their performance parts because you can go to summit or jegs and get the exact same intake/carb/cam/rockers/etc. for half what they charge.