What transmission to plan on using?

Starting the planning process for an upcoming project and am confused about which transmission I should look into using.

The car is a 71 Challenger, /6 with factory 3-speed manual. The engine I am considering buying (if they guy still has it once I sell either of my 340 Dusters) is a 2018 5.7 that has upgraded cam, springs, lifters and injectors. It Dyno'd at mid 400hp range to the wheels in the 2018 Challenger it was pulled from (guy upgraded to a 6.2 and is putting a whipple on it...

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