you can put anything you want into a z, im always screaming v8 is the best, for cost wise to performance (hp to $) i also am a big fan of the multi caburated japanese engings, i just drove my boys 71 corolla with twin sidedrafts, and that little thing will hit the tightest doughnuts known to man.
the v8 is very well documented, which adds to cheapness, everything is mapped out for you from start to finish, and once you have your template swap finished, you can get real jiggy with it. my second favorite is the gnx swap, which makes awesome power, and its turbo, just might be my next drivetrain, its the bomb, with BOOST. it all depends all your skills and mouthpiece manipulative capacity (read G A M E).