In my opinion, the most accurate gauge will be your brain...no wait, let me explain:
When you fill up set you tripodometer to zero. The next time you fill up, divide the tripodometer miles by the gallons that you pumped into your car (I did it on the back of an envelope, but of course you can use a calculator). That will give you your average MPG for that "tank" of gas. The more times you do this calculation, the more insight you will gain as to how efficiently your car is using gas (also considering how aggressively you drove during that "tank"). After about 4 or 5 "tanks" of doing this, you can have a pretty good idea of your car's fuel consumption rate. Now when you compare your car's fuel gauge level to the tripodometer number, you will be able to judge with surprisingly good accuracy how many miles you have left before needing to fill up, and have a good idea if you are getting "good" gas mileage at any level of your gas tank. Of course, the more you do these things the more intuitive you will become.
I suppose I feel compelled to say this because my experience with the type of gauge that you are asking about in other cars has led me to believe that while they are neat to look at, most (affordable ones?) are only capable of calculating an instantaneous MPG statistic (that can fluctuate greatly depending on how hard one accelerates at any given time). Consequently, you could spend the money on endlinks or something else that provides a better "bang for your buck."
Just some food for thought, not trying to rain on your parade. =)
70 to engine rebuild faster than you can say "poofff!"