have a 65 mustang 289--block was bad--machine shop says a 302 block would work.now need to know which flexplate and harmonic balance to use.

 

If you're using the stock 289 crank, all your 289 stuff will work fine. If you get a late model 302 block, you can still use your 289 parts but the crank will have to be machined to use the one piece rear main seal.

 

The motor is built--machine shop says it will take 50oz flexplate--bought it from them and it hits the starter and the backing plate. He says the reason for the 50 oz is to make it idle smoothly. would u go back to the orig 28oz or stay with 50oz? Could it be a bad 50 oz?

 

If it's a 289, it needs the 28oz flywheel and balancer, not the 50oz. If you have the 28oz balancer and 50oz flexplate, you're going to have all kinds of vibration. Unless by 302 block, you mean short block. Are the internals from the 289 or a later 302?
They make a 157 tooth and a 164 tooth. You may have the wrong size too.