Well the trans in the 72 Ford is F'd. 3200 miles
Started having issues of it not returning to 1st at a stop about 5 weeks ago (on camping trip of course). I checked everything I could including a fluid sample. Otherwise trans performed as expected. Stayed nice and cool with huge cooler.
Having been through similar issues before I figured the fluid is contaminated with particles causing the pistons in the governor to stick most of the time but not all. So I drained and pulled the pan. Not good.
I have 2 options
a) Fix the C6. Replace cooler, torque converter. Around $1400, Truck down for 4-6 weeks
b) Go to a 6R80/100. Used trans, adapters, controller, driveshaft mods, crossmember mods, replace cooler, new lines. $4.5-5k, 12+ weeks
6R has a 4.17 first and 0.69 6th yielding a 6:1 gear spread. C6 is 2.40 first and spread. So first would be geared down by 1.73 which is
almost like having a doubler. It will burn the tires off like it does in 2WD low now. The relatively shallow 2:1 of the NP205 is offset with a steep 1st.
Overdrive of 0.69 puts 75 MPH cruise at 1800 RPM with help from lockup converter. 5th is 0.87 and would be 2300. I think the lockup converter will help $/mile and increase range. Thinking a used low mileage trans that has never been opened or touched anyone but Ford.
I'm afraid plan a will result in same situation like the c4 in my old bronco (3 times)