70 Mach 1 - 351C - what rearend did it come with?

With the Mach package and 351, I'll go out on a limb and say with confidence that the 9" was standard.
Gearing is another story.
There would have been many available ratios, so you would need to check the tag or count the turns to know for sure.
I would bet that 3.00, 3.25, 3.50, and possibly 3.91 would have been available with that combo in 70. All up to the buyer, however tranny selection could have narrowed it down some.
 
All Clevelands got a 9 inch. Non-Mach's got ratios as high (*or low) as 2.75:1. The standard ratio depended on the transmission choice. All the ratios you listed were available, as well as 2.75.
 
generally speaking:

2v got the 2.79 or 3.00
4v got the 3.00 or 3.25

then there were other optional ratios such as 3.50, 3.70, 3.89 and 4.11 but it's unlikely you'll have any of these as they weren't very common.
 
I read somewhere that the 4v Clevelands came with a nodular. Is that correct?

I would say no. My 9" came from a 351C equipped 70 cougar XR7. I pulled the entire rear myself. Its not nodular. Its 3.00:1 open gears and 28 spline axles. It looked to be original to the car.
 
The "Boss" and "CJ" packages came standard with nodular rears, but they can be found in just about anything. I've found them in everything from 4 X 4 trucks to vans to grocery-getter station wagons.

True.
Ford was kinda funny about the rear end stuff.
N-cases would pop up where you wouldn't expect, yet some cars that you would think they would be in they just don't... K-code for example, or Cleveland cars.
Alot of I6 cars got 9" rears when the same car with a v8 would get an 8". :shrug:
(grocery getters, not talkin' Stangs)
 
nodular cases were also common in 4x4's and vans, even some big lincolns. there was a method to the seeming madness, heavy duty applcations got the nodular cases so even cars and any truck with a towing package could get one.