well im posting this for my friend on his 68 covertible he has a stroked 357 putting out 620 all motor, so he asking me if there are any automatic transmissions that can hold up to 600+ motor and also what would be a good radiator

 

A built C4 or C6 should hold up.

 

Go with a beefed up C4, you will loose less HP through it, than through a C6.
As for a radiator, a 4 row will work fine.
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