First, what does a vacuum gage show during the missfire?

The bottom line here is I can envision any number of reasons for this. Proper testing will reveal the cause.

Trouble code PO 306 for 1996 reads:
Cylinder 6 Misfire Detected.

Could be a number of things. A couple of suspects are spark plug or spark plug lead.

Here is a link that might be useful: Code PO 306

Everything has been changed including the injectors. scan
tool shows #6 misfired 3500 times. Other cylinders misfired
any where from 35 - 100 times also. there is nothing left to fix. The engine light comes on at first start, sometimes. The engine stumbles, light comes on. engine light slowly goes out. Yes it dims out. as engine warms to
normal it will stumble again. Light dimly goes to brighter
Never seen that. Light usually come on or goes off. this
truck has a centerlized injector. Looks like a spider sitting on the manifold. replaced by G.M. they said that was the problem. NOT. I guess this guy will have to live with it.

By centralized injector, are you referring to a throttle body injector?

Does this engine have an aluminum intake manifold nested between the two v-banks of cylinders? If so, check the tightness of the mainfold bolts. Put a wrench (or socket) on a few of these bolt and attemp to turn in the tightneing direction. Are these easy to turn, i.e., not tight? During this test, do not turn the bolts more than 1.4 turn else you may warp the manifold. It has a specific tightening sequence much like an engine head. If you find that more than one bolt is not tight, it was likely leaking and needs a new gasket set.

Refer to a repair manual for installing a gasket set.

I believe there are coolant passages in this manifold. check the condition of your engine oil. Pull the dipstick and look at the oil. Does it have normal color and appearance, or is it sort of brown and a little syurp-like and has the oil level risen? - if so, coolant may be leaking into the crankcase via the intake manifold gasket.

On an engine of this age, check all of the vaccuum lines and fittings. Do not interchange fittings because some of these may be ported (have built-in orifices of specific restrictions).

[John_G would likely make a variety of vacuum tests as part of his diagnostics. Me - I'm only making blind suggestions based on what might be the matter. It is too difficult for me to know what to look for without seeing the engine and its behavor, and without a repair manual that shows the details of the engine control layout.]

Hi Jem.

Centralized injector means this has a single fuel injector, and then eight spiders with poppet injectors coming off of that to direct the fuel right to each cylinder. The poppets require a pressure differential of 56PSI to open. One of them could stick open or closed, and have an impact on that cylinder, so this engine could be lean on just one cylinder in this case, which would be #6.

The reason I asked for a vacuum gage is I want to know if the valve train is truly functioning correctly or not. A valve can close fast enough to give good compression test readings when cranking the engine. But when it runs just a little faster, and heat from combustion comes into play the valves can stick ever so slightly. It';s quite common to occur in the center cylinders on each bank, and will occasionally move around the engine as the "missfire" causes exhaust gases to be leaked back into the intake and have an effect on a companion cylinder.

As always, test, don't guess. There is no sense pulling the heads righ now and sending them to a machine shop unless this issue is completely confirmed.