Tune seems to last about 15,000 miles or more. I drive mine daily and can not seem to keep my foot out of it. Electronic fuel injection for this 72 model. Yours may have something else. MPGs on mine is about 14-15 around town and 19 hwy, but I have a very short gear, 3:90s and tacking about 3500-4000 cruising. I understand that the 450 (4.5) is in some cases not as reliable as the 3.5. 4.5s easier to get parts on. Ride will be kind of like a corvette or transam. I get the feeling that the SLCs are going to raise in value due to the stronger bodies and we can all carry kids in them. Good solid cars, made to last, if taken care of.
Hope I am of some help,
Best,
TSL