Congratulations on the weight loss. I'm a bit younger (43) but started the year at 300 and got down to 220 following a low carb diet combined with Medifast when I got lucky and found a link to Rippetoe and SS. I am now up to 225, but my bodyfat has fallen from 23% to 18-19% (down from 37% when I weighed 300).
As far as chinz go, check out the "Art of Masculinity" video - How To Do More Than One Stinking Pull-up Routine -
https://youtu.be/eT6jSZhMyQ4. A variation of this took me from being unable to move my body when hanging to a real chin-up using a negative chinup approach (eccentric movement). Now following this, I can do 6 sets of 1 and then I try 3 sets of 10 negatives. I was surprised how effective the eccentric movement helped the concentric one.
Good luck and keep up the great work.