swapping 5speed to Auto for race purposes only

me having stupid ideas again.
Just for some race Weekends i want to swap out my Toyota 5speed for an automatic tranny. And swap back afterwards hahahaha
Got a deal for my MT Semi's i don't use anymore to get a 727, 904 or 998 core to rebuild.. Which one is best in your eyes? 998 i don't know at all. 904 is not heavy but can it survive approx 400hp on Slicks on prepped tracks? 727 is strong but heavy.
what is the best adapter Flex plate to use?
everything else is more or less Standard. shifter, cables, hoses etc

what converter are you Auto guys running with a rear Ratio of 4:10 or smaller? Need to drive 150mls on own tires to the track...

regards from Germany,
Michael