Miss - No Load Driving & Reverse

[LuvNLife]

OK; my money gun is almost out of bullets, so I need some help brainstorming a problem, rather than tossing random parts at it.

'85 C10, with a new Chevy 350/260 long block. At idle, it's smooth as can be. Drop it in gear, same. Accelerate lightly, and I begin to hear a random miss; don't really feel it bucking or anything, I just hear it. Driving steady state at like 25 mph, same. Pull out on a main road, throttle response is fantastic, half pedal and it pulls great, with no hint of a miss. At steady cruise, 55-60 mph, runs and sounds great. So long as there's a load, like a small hill, it sounds great. When I drop it in reverse to back up my 150' inclined driveway is the worst. Not only do I hear the miss, I feel it bucking. The truck also smells pretty rich at idle, but zero black smoke.

Here's the hardware I have: engine with less than 200 miles on it, Edelbrock Performer EPS intake, 1/2" wood carb spacer, Edelbrock AVS II 650 (no choke) carb, fuel pressure regulator set to 4 pounds, Patriot shorty headers, 10mm new plug wires, R45TS plugs, new (inexpensive) HEI connected (now) to ported vacuum, valves lashed to 1/4 turn past stopping of "clacking". The trans is a Turbo 350 with about an 1800 stall converter (was in the truck already).

Thoughts and recommendations would be appreciated.