Opinions needed, change the ignition timing?

[Tom]

On my 79, Ive been trying to get the most gas milage possable. I found that happend with ported vaccum, and 10* initial with the vaccum pluged off. I can manage 18-20mpg on the freeway if Im carefull and traffic allows. This is with the 3.07 rear I just found out I really have instead of the 3.42's I thought it had. I also really need 89octane with the summers heat even though this vortec motor is a 87octane motor. Now I know vortecs use less timing then normal heads because of the combustion chamber, and the stock vaccum advance can gives WAY too much advance, so I was thinking of messing with the timing using a crane vaccum can. Im hoping I can get it back to 87octane and maybe even more milage, but I REALLY dont want to kill the milage Im getting now. So what do you guys think, should I try the crane can and manifold vaccum, or just live with 89octane?