First is the conventional use of plus development. N+1 (extra) development is conventionally used with reduced exposure, say EI 200 on your FP4. N-1 (reduced) development is conventionally used with added exposure, like your EI 64.
Are you really are using plus development (more than normal development time) with EI 64 exposure?
Second is FP4's normal latitude. Shooting FP4 at EI 64 and developing normally is easily inside the limits for getting a great print. All that normally does is make more shadow detail available on the film. Using normal development here should not be a big problem. With FP4 I've used XTol, D-76, DD-X, and WD2D+ and using normal development times for all of those developers I can get nice negatives shooting at EI 32, 64, and 125. With DD-X I can even shoot FP4 nicely at 250.
Given that, how was over development judged? By that I mean did it take grade 00 paper to get the middle tones to look right or ... ?