Does anyone have any suggestions what would make my lt4 engine vibrate when I accelerate? It will vibrate at around 3000 rpm. I have had all tires balanced. Could it be my clutch and pressure plate?

First you need to determine if the vibration is the car or the engine. Here is how to start checking.
Tires out of balance will not fix a engine vibration.
1) does it vibrate at 3000 rpm when parked?
2) when driving at the speed and rpm of the vibration push in the clutch and let off the gas see if the vibration goes away, if not then its the car not the engine.
If it does go away then it may be the engine and you need to start looking at the easy stuff first.... engine and tranny mounts, look and see if they are oil soaked which makes them sag and weakens them. If they are ok then I might start looking at the harmonic balancer, maybe its loose or cracked, possably a warped or out or balance fylwheel but that is a bitch to check on that car so I would attack that last.

If the vibration is the car... wheels being balanced does not rule out the wheels yet, you could have an egg in one of the tires that doesn't throw the wheel out of balance but still affects its rotation. You could have an out of balance driveshaft.

Try some of these tactics and it will help you narrow the concern down.