Basic and inexpensive is a distributor re-curve. Find out how much initial timing your engine likes and will start with when warm. Then shorten the advance curve to limit mechanical advance in the area of 36-38 degrees at 3000 rpm. Then you can work with the vacuum advance to get a total not to exceed around 45 degrees at cruise. More initial timing will give you an immediate and noticeable improvement if the car is running with less than it desires. Just make sure you aren't pinging with poor fuel quality or too much timing. The 273 has closed chamber heads that will swap over to the 318 and give it a nice boost in compression. You can usually find a good 318 pretty cheap and it is a bolt in swap.