399 horsepower 1965 389 with factory AFB intake and carb running 3-tube headers:
https://forums.maxperformanceinc.com/forums/showthread.php?
t=836191
As Stan has already pointed out the log manifolds might be an issue you’re going to have to work around. The use of three tube headers in the link above might give you some hope that it’s possible with reworked log manifolds to achieve 400 horsepower or get pretty close.