Everything depends on your budget and how far you want to go. Has the engine been rebuilt before? If not, how does it sound? If there are no "funny noises" and a rebuild isn't in your budget, I would just do an intake/carb swap. That, and a good rear end gear ratio should make a world of difference.
As far as carbs go, I wouldn't go any more than 600cfm on the stock cam. If you do go with a rebiuld, DON'T ditch the heads. The 2v heads are more liveable than the 4v heads. Just have the exhaust ports worked well and a real good pocket polish in the valve bowl.
I hope helped out a little. Good luck.
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