Ames Performance and Performance Years both offer the mounting "adapters" needed to install the earlier blocks in later cars. Not a cheesy "fix", either.
For all intents and purposes, 389 has exactly the same potential as 400. The only parametric difference is the bore size, .060" smaller. Cranks interchange. Using 400 heads from the late '60s makes 400 HP a slam-dunk. Good cam, induction and exhaust and you're "there".
What Lynn (LATECH) says about the starter mounting is correct. All '64 GTOs had the "block mount", but most of the "big cars" still had the bell-housing mount. NO problem is "insurmountable", just impractical.