10 hours and one good night's sleep later, just and purely as a suggestion, I am going to propose that you swap out those 20 inch wheel/tire's and replace them with some 16's, just to see what difference the shorter tire will make to you initial acceleration.
What I am thinking is that the 20's are too tall a tire for your level of engine build. Not going to get too far into the physics and mechanics of circular rotation and motion but the bigger the tire/wheel, generally speaking, the greater the amount of inertia or resistance to movement that has to be overcome. For a car, torque is the force, applied to the wheel, that overcomes its tendency, aka inertia, to want to stay at rest.
Your choice of motor, a 350, even if built up from stock, has more of a reputation for horsepower, or speed if you will, than for torque, or low end acceleration. It's strong point is its ability to build rpm quickly. It was it's bigger brothers, the 396/427 that were the torque monsters of their era, along with the big block BOP's, and their counterparts across the aisle in Ford and Dodge country.
So, when you whack the throttle pedal, your motor does its best to initiate motion BUT, it has to overcome the weight of the vehicle, the parasitic horsepower loss throughout the drivetrain, plus the combination of rear end gear x Tire height/circumference and, it just might be proving too much workload for the engine. It, the engine, is telling you this by displaying lousy acceleration and bogging when you go to WOT.
Even at cruising speed, when you try to suddenly accelerate, the amount of inertia or resistance to change in speed is still high enough that the vehicle does not respond as quickly as you would wish it to.
You have the acceleration of a 10 T Dump truck (which uses a double transmission combination to overcome inertia and weight) when you want the acceleration of a Ferrari.
With the 16's mounted and secured, even a lap or two around the hood or a short blast up the highway ought to give you a feel for whether the vehicle is responding more enthusiastically to input from the go pedal.
Nick