Based on the charts LockDoc posted in the following thread, it looks like HC is 3.73 ratio with posi. 1970 was a transitional year where they went to a wider overall rear end width. Since the HC code is listed in the 1969 chart, I'm wondering if your housing might be the narrower design. I may be wrong about this, but I believe the wider design had GM54 cast into the center housing, whereas the narrower design was GM51. That's how it was on a wide 1970 housing I sold recently vs. a 1968 housing I purchased to replace it. Measuring the width of the rear end would be the conclusive way to tell.
https://www.67-72chevytrucks.com/vbo...d.php?t=578793
Copied from the thread below:
axle width is 55.5 wide with 30.5 long axles on the narrow rear end. earlier rear end
axle width is 57 wide with 31 5/16 long axles on the wider rear end. later rear end
http://67-72chevytrucks.com/vboard/s...d.php?t=640514