Archaeological discovery!

[Dr.Jass]

I would think the '75 would have better jetting, particularly if you've got a 440. The 400 carb would likely be a tad leaner.

The blower resistor is right on the heater box, four terminals with what looks like a broken-off fifth (that's how it's supposed to look) held on with two bolts and clearly marked "3502570".

 

[71ChargerRT]

That's what I thought, I have a couple I can locate. I probably have a couple more somewhere.