In order of importance:

1) Tune what you have really, really well. Carbs and distributor must work like new. Do a really accurate valve adjustment.

2) Much larger exhaust pipe, retaining the original manifold and downpipes. Run this straight under the axle instead of looping it around. Muffler goes as far back as possible, preferably between the gas tank and quarter panel. This is better than the cheap headers being commonly sold.

3) Shave the head to bring compression up from 8.7:1 to 10.0:1. That's 115 HP instead of 90 right there.

4) Volvo D cam instead of the stock C cam.

After that, you're looking at real money to do anything effective.

A heavier front sway bar is very, very worthwhile after you've ensured the rest of the suspension and steering stuff is 100% good. A front/rear set is nice for sporty driving with limited power available.