Bought a kit from eBay.After I installed it when I rev it up it misses.Seems to run best at tdc if I advance it more it really misses.Any ideas?

Balancer slipped and timing mark is off.

I think MSD's will do that if the leads to the distributor are reversed. What does the wiring look like? Can you post up the diagram?

Tore it down to check the timing chain.It hadn't jumped time but the chain was really loose with a nylon gear so it needs replaced.Everything us wired correctly per the instructions.

How did it run before the change?
I suspect something with the install (wiring or defective part) if all was well initially.

bring the dampener to 15 BTDC #1 compression then turn housing till the tooth is lined up with the magnet & the rotor is under or near under the #1 plug wire. if no go then as said reverse the leads & retry.

it detected is a Chebby product so the car decided not to work

More likely, "it detected a China-product so the car decided not to work"

If you got some China ignition-stuff (ECU), that is usually crap and can break up when revved.
What did you buy exactly?

To vague a question. More details would help. Nothing wrong with a good HEI unit. We bought a cheap one for our endurance car. Worked great until we hit the track and the gear stripped ran like canned ass over 3500rpm. Dropped the stock 160k unit back in and it ran great. If you bought some junk then it is to be expected. If you hoked it up wrong again, expected. Look at what you did and re-do it all over again. A simple bad ground can cause major issues,

CHECK THE WIRING!!! I chased my tail for several hours on my Distributor machine as the pick up coil leads to the HEI module were backwards (long story) and it caused some real strange timing events.

As stated wired up per instructions.

I would Verify your grounds are good and then try another module. You could also try a few jumpers from the battery to the module / dist ground . Only thing left after that is the pick up coil itself.

Got it back together with the new timing chain Redid the ground.Ran fine for awhile then started missing and breaking up.Vacuum gauge us moving a little also.

Do you have vacuum advance hooked up? If you do, try capping it off. I've had the vacuum advance cause misfires/breakup if the rotor phasing is off.