I have a 70 Valiant with a 904. What's the difference between it and the 727? Would the 727 be a better trans behind the /6? Would it improve MPG?

How do I know what tail gears my Valiant has in it? I really don't know much about tail gears so I need a little education. Is there a good gear ratio for MPG? Would the trans and rear end from a 74 Valiant with a 318 be a better setup?

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70 Valiant Super Six

727 is a heavier-duty transmission. It is larger and heavier, and thoroughly unnecessary behind the vast majority of slant-6 installations (the 904 even in stock form is quite sturdy, and can be beefed up considerably.)
No.
Jack up one rear wheel. Mark the tire. If you have an open rear axle, turn the tire [two] complete revolutions (Sure Grip: Just one) while watching the driveshaft and counting U-joint caps. There are four of them per revolution, so I just count the number of U-joint caps that face the ground over the course of two (open) one (suregrip) tire revolution, then divide by four, thus:

16/4 = 4 = 3.91
14/4 = 3-1/2 = 3.54 or 3.55
13/4 = 3-1/4 = 3.21 or 3.23
12/4 = 3 = 2.93 or 2.94
11/4 = 2-3/4 = 2.71 or 2.76
10/4 = 2-1/2 = 2.45
9/4 = 2-1/4 = 2.26
There's no one single gear ratio that's best for economy. The right gear ratio depends on your car's engine configuration, weight and equipment, your driving conditions (mostly city? Mostly highway?), your tire size and other factors.
A 318 trans won't fit behind a slant-6 without an expensive adaptor plate, and there would be no advantage. The 7-1/4" rear axle from a '74 interchanges, but is a weaker piece than the 7-1/4" in a '70.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks a lot Dan, that's very helpful.

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70 Valiant Super Six