I bought a 383 stroker motor that is externally balanced. But the problem is that it has a balanced flexplate but my car has a T5 transmission, so my question is it possable to use the damper to find the right balance on the flywheel?

 

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The damper is what it needs to be; doesn't have a lot to do with the opposite end of the motor.

However...

The correct "unbalance" for a stock 400 flywheel, is real close to 25 oz-in, pretty much in line with the funky-shaped flange thing on the crank. Which is, in fact, the rearmost counterweight for a 350.

Easiest way to get what you want, risk- and trouble-free, is to have a stock 83-85 Camaro/Firebird 305/T-5 flywheel, "unbalanced" to the stock 400 specs. That typically requires drilling about 20-ish ½" dia holes about ½" deep around the outer edge of that flywheel, directly opposite the dowel pin hole in the flywheel. (dowel pin hole is in the center of the counterweight in question) Your professional balance shop will have no trouble whatsoever doing this.

Easiest way to accomplish it, is, to bolt a stock 400 flex plate and the flywheel you will use, back to back, except 180° out; and neutral-balance the combo.

You could use one of those "pork chop" weights also; sometimes that works out just fine. Other times, it affects clutch operation. It's not a "bad" idea, or any such; just, it involves some risk. The way I told you though, is risk-free.

Cost is usually about the same either way.