Slant 6 Help

Sounds like you are young and just starting out.
Learn how to tune the slant to run the best it can.
Lubricate everything, suspension, u-joints etc, a 40 year old car needs attention all the time, make it reliable.
For a little get up and go swap rear ends, for something like 3.55 or 3.91, not cheap unless you find a doner car.
The slant is some what easy on fuel, so that means more money in your pocket to take the girlfriend out, etc.

FYI the same car with a factory 318 is a dog is stock form, so work with what you have, make it safe and reliable. Horsepower can come later.