here's the deal - Only 2 ways to increase HP across the rpm range - more cc's or higher comp ratio. everything else RESHAPES your power curve - taking from the low rpm's and adding to thr high.
Since you can't change the displacement easily - raise the comp. ratio. Your engine probably has dished pistons - a set of flattop pistons and a low cc head should give you 10:1 - about tops for a street engine w/o a wild cam. That puppy will run then...
I like the 83' Maxima head since it has a redesigned combustion chamber with a "squish" area the earlier Z heads don't have and it fits right on.
good Luvk
GG