Howdy

I have a 60 bird parts car that came with a 390 and an AOD adapted to it.
What are the odds that adapter, converter, and trans will fit behind the original 430
in my 59 Bird?

Hello,
If your 59 Bird 430 has 6 bell housing bolts it is the same as an FE and will work with FE flex plate. Bolt up and go. Just adjust or make a new kick down to fit the taller MEL.

OK thanks. Still have to pull trans and see if 430 starter will
work with bellhousing adapter. look like kit comes with flex plate,
https://transmissioncenter.net/shop/ada ... -fe-motor/

so also not sure if I can use 430 flex (if they are different). Is 430 internal balance?

430 is zero balanced. It all should work unless your kit required some some block modification. I have an FE FMX behind a 59 430.