ok so i should clarify that everything is stock apart from the intake (edlebrock performer) and carb (holley 750, built by davinci). It is definitely a bit on the rich side.
so without further ado: Stock 350, mildly low compression on cylinders 7 & 8. pontiac automatic (2 sp), 3.23 diff, running marine fuel (full oil, 93 octane), petronix ignitor 3 with recommended coil and gapped plugs, 172 degree thermostat, PVC relatively recently replaced.
Cant speak to RPM, no tach. Same for temps, only a warning light.
OK, stock 350 2-Bbl engine. Factory 9.2 compression/265 HP on a good day. First, the Holley 750 carb is too big. So it is going to run rich and you are probably giving up throttle response. Yep, you can jet it down, but still too big on a stock engine. The engine also does not have the cam to take advantage of the bigger carb. A 600 CFM would have been a much better selection.
Mildly low compression on 7 & 8 is definitely not a plus and you are probably pushing some oil into the cylinders. I'll also bet the valve stem seals are shot and your vavle guides could be worn, both can allow oil to enter the cylinder. If after the engine is shut off and sits for a bit and you fire it up and you experience that puff or blue smoke out the back - seals/valve guides are in need of attention.
Hopefully, the timing chain & gears have been changed out. The nylon cam gear is notorious for chipping and breaking off teeth and the chain will get sloppy. Once you start pushing past about 70,000 miles, it should be changed out. If it was a GTO w/400, I would not go 60,000 without changing it as they wear faster due to higher valve spring rates and most people beating on the cars. That said, if the chain & gears are sloppy, it will affect your timing accuracy and trying to adjust your initial and total timing via the distributor will be almost impossible as it will move around on you.
2-speed trans is not ideal. Low gear has a 1.76 ratio as opposed to a 3-speed automatic having a 2.52 1st gear ratio for much better acceleration. Takes more HP/Torque to move the car with a 2-speed on take-off and through the entire Lo range - so this puts a load on the engine and over carburetion is a bad thing.
So, in my opinion, your selection of add-on parts are incorrect for your engine. Aggravating the situation is a worn engine. The 350CI is a fine engine and has build-up potential, but adding components to a stock low-performance engine has to be done carefully and with matching parts. Many will go too big, which in my opinion has been done here, rather than add parts that are better suited and often not what you want.
If you have to use the Edlebrock Performer & Holly combo, go with a smaller carb like a 600CFM/vacuum secondaries and dial it in. Check the chain/timing gears for excessive play and replace if needed. Dual exhaust is needed to improve exhaust flow/breathing. Dial in your timing to maximize engine HP/Torque.
Without a cam change, you are not going to get big HP/TQ results over a stock 350CI by going to a bigger carb. The aluminum intake saves weight, but is no better than the stock intake so no real gains there. You might be adding 15-20 HP at most using a carb matched to the engine.