Did the gearing in the transmissions between a 66 Valiant (904) and a 72 Dart (904) stay the same? Could I run the 72 by just connecting the center output of the neutral saftey switch?

The main change was the '68 and up got a bigger input shaft and pilot on the converter. The late converter will not fit in the register on your '66 crankshaft.

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...and no, the gearing is the same in the '66 and '72 transmissions.

In '67, the 904's for slants gained the part throttle kick-down feature. I believe it will work on the '66 transmissions. I'll have to break out my shop manual to explain it, but its basically just a mini-valve body that attaches to the side of the valve body. Its already on the '72 you have. Perhaps somebody can explain in greater detail for you.