well you failed to post a great deal of useful info here, part casting numbers, and a great deal of related info, at this point I don,t know if your engines a 366 ,396, 427, 454 or a 496, I don,t know what heads, intake,ignition , cam, or compression ratio you have or the transmission , stall speed, rear gear ratio,but assuming it is a 496 engine the first things ID verify are basic function, ID verify the block and head casting numbers, and if you can, get any info on who built the engine and what parts were used.
ID do a basic tune-up with new plugs, and a new rotor distributor cap, and verify the engines firing order is correct and check the ignition wire with a multi meter, ID re-adjust the valves and verify the cam it has installed in it!
a restrictive exhaust, clogged catalytic converters, weak valve springs, a weak fuel pump, or a badly timed ignition all instantly jump into mind,
next check the sub systems, like are you getting consistent fuel pressure, and check your not dealing with a restrictive exhaust system, have you checked your ignition advance curve, and have you done a compression check?
obviously you might have weak valve springs or a worn cam, so that will need too be checked, and it would help a good deal if you posted a great deal more info,
once were 100% sure you have a fully functional 496 , and not a 325 hp 396 with a 100K plus miles on it, we can look into the details in much greater depth.
obviously the car did not come from the factory with a 496 BBC , so you'll need to verify what you have installed in the engine bay.
1968 Chevrolet Chevelle Engine Options
Letter Code Engine Horsepower
ED 8-396 325
EE 8-327 * 275
EF 8-396 * 350
EG 8-396 # 375
you could pull a cylinder head and use a caliper to measure bore and stroke
but its faster to pull a spark plug and measure the cylinder volume with a burret and some oil, at BDC
a 496 should have about 1134cc cylinder volume at bdc
remove the rockers on one cylinder and rotate it to BDC to test, your results won,t be exact because the cylinders at an angle and it won,t completely fill during the test, some trapped air will prevent it completely filling
496 /8=62 cubic inches times 16.387-1016 cc then add 115 combustion chamber volume=1131cc and add a couple for head gasket volume.
if it measures significantly less your 496 might be a smaller engine
example a 396 would hold a bit less than 926cc
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