'69 C10 327 SBC Off Idle Stumble / Misfire Help - I feel like I've tried EVERYTHING

[SRT8-X]

I'll start with the basics; I have a 1969 C10 that has been going through a frame-off / nut and bolt restoration for several years. Below this introduction, please find the specs for the engine and drivetrain. I apologize in advance but my summary and narrative will be lengthy as I want to provide as much information and detail in hopes that someone has an idea of what may help.

Engine:

1965 Chevy 327 CID
Stock internals
Stock double hump “462 fuelie heads”
NEW Edelbrock Performer EPS Intake
NEW Edelbrock Performer 14063 600 CFM carb with electric choke
NEW Edelbrock Perfromer RPM 1721 Fuel Pump (reads 4.5-5.5 PSI on gauge)
NEW Edelbrock billet fuel filter and fuel line
NEW AC Delco GF61P in line fuel filter & steel fuel lines from tank
NEW Hamburgers 3225 1” aluminum spacer
NEW Performance Distributors / DUI HEI vacuum advance distributor with 50k volt coil and high dwell Dyna-Module, advance from timed side of the carb
NEW Performance Distributors / DUI C9051BK Live Wires under header 8mm wires with heat shield
NEW Autolite AP86 Platinum Plugs
NEW Ceramic Coated Shorty Headers
NEW 2-1/2” stainless exhaust with Flowmaster 40-series mufflers turned down at rear axle
NEW Billet Specialties breather with integrated PCV valve
Original GM in cab tank with sender
NEW Derale 16927 4000 CFM dual 13” puller cooling fans and shroud
NEW Full Painless wiring chassis wiring harness, electric fan harnesses fans on 195 / off 175
NEW 180 degree thermostat
NEW Lokar throttle cable conversion and kick-down cable
NEW Optima battery and true 0 gauge oxygen free copper cables
All gaskets are FelPro

Transmission / Drivetrain:

Mildly built TH350 trans with stock stall converter, B&M Trick Shift fluid
NEW Derale 14200 3-1/4” deep pan with cooling tubes
NEW Derale 13900 650 CFM Transmission cooler mounted under passenger side of cab
NEW B&M modulator with steel modulator vacuum line sourced from T-fitting at rear of intake
NEW aftermarket CPP Power Steering
NEW aftermarket Wilwood Master Cylinder / CPP Booster Power Brakes
3:73 stock rear differential with one-piece driveshaft, NEW Moser 30-spline axles and Bendix drum brakes

The problem is once the truck is warmed up, it begins to stumble or surge off idle, often misfiring from off idle to about 2500 RPM making it frustrating cruising around town. It runs OK cold but not great. Once it reaches 2500-3000 RPM, the misfire clears and it runs great. The kick-down works great and the truck is a blast 3000 RPM and up. However, off the line, even if taking off slowly with minimal throttle or flooring it, the truck does not go as it should, sputtering and popping, hesitating, etc. It has never been bad enough where it stalls though. WOT from standstill does not do much more than limited throttle off the line. Cruising and WOT is different however as it's further up the RPM range resulting in good performance. As noted, it runs great cool, idles great both cool and hot. It also does have an intermittent small miss at idle at times as well known by a vibration and a vacuum gauge. There is not enough power to take off well much less to spin the tires off the line or power-brake (under load).

The engine has not had any internal work or inspection since installing a few years ago even though we've only put about 400 miles on the truck since getting her roadworthy. The engine oil is clean and has Royal Purple 10W30 oil installed, K&N screw on filter and a touch of Marvel Mystery Oil. It does have a small transmission leak at the front seal / pump but shifts fine and does not show any signs of internal damage. The truck has an “under load” intermittent vibration at 40 MPH + that goes away if put in neutral leading me to believe there could be a converter balancing issue. We run 93 octane pump gas with Lucas ethanol treatment and Marvel Mystery oil at every fill up. I started using the Mystery Oil at the suggestion of a buddy who believed we may have a valve sticking. There is some valve train noise at times but it’s not significant beyond anything I’ve heard with other SBC engines.

Other than the new parts, I’ve tried tuning the carb for rich / lean settings, tried several variations of metering rods and springs, several choke settings, several idle settings, all (3) accelerator pump rod settings. I have not replaced or checked the accelerator pump itself or the floats in the carb, as it was new in the box prior to install. Vacuum registers about 20” Hg at ide but does have a -1-2” fluctuation at idle sporadically. The engine timing is set at 16° initial at idle and 36° total timing @ 3200 RPM. This has been verified several times and reset, adjusted, reset, etc. I have put heat insulation boots on the two rear plug wires due to their proximity to the header tubes and noticing an arc on cylinder #7 even with the live wires. The new live wires have white residue from resting on the headers but they are supposed to be rated to 1200 degrees. I have not seen any arcs here under darkness but some plug wires are very tight against portions of the headers due to the run being short on this custom set. Compression seems good based on feel but I have not tested each cylinder. I have not pulled the valve covers to set the lash but have physically observed the rockers and springs.

No parts, other work or tuning have made a significant change for better or worse. I’ve done all the work on the truck myself with the exception of the upholstery on door panels and dash pad so I’m opposed to trusting a local shop or others to try diagnosing this situation and don’t have an open checkbook to do so. My father was my mentor, this truck was his daily driver for nearly a decade in the 90’s and I’ve built it as a tribute to him after his passing in 2016. I’d really like to complete this and have a solid running truck as it’s been meticulously built otherwise. Please help if you can and if you have any ideas. Thanks in advance for your time if you’re able to reply with any suggestions or comments.