Fortunately I'm 5'8 and not obese (yet - office snacks are killing me)
It's funny you mention freezing - I was actually going to ask if it was too hot! I as on a VS flight from SFO-LHR and it was sweltering. Thanks for the reminder to layer up!
And as for people suggestion different routes - unfortunately that's not an option due to a variety of reasons, some of which were out of my control.
At 5'8" you will be ok in E+. UA runs it's planes cooler (Asian airlines, particularly the Japanese tend to run warmer airplanes, SQ is the exception to this rule) so a blanket/shall for the legs and a sweatshirt WITH A HOOD you can use to block out noise/light is very useful. I tend to wear shorts (I find long pants get sticky after 16 hours....) and a combo of a blanket and a good hoodie does the trick. Neck pillow as you can't rely upon the headrests, and there is too little recline in Y. Also I would bring a face mask to block out the light. Noise canceling ear phones are also a help.
Then have a couple drinks and eat before boarding if you have lounge access. I prefer evening arrivals (SQ's flight gets in not at 8 am but 7 pm) to Asia, but with an 8 am arrival your strategy will be a question of how you sleep in airplanes. I can't sleep more than a few hours at a time in Y (I do better in PE or J) and my strategy usually is to take a nap right after the meal (on UA you can do right after boarding, skip the meal) then wake, then try to sleep again in the 6-8 hours before landing.
But I would definately try to get as much sleep in the last 8-10 hours of flight with an 8 am arrival, which probably suggests staying up as long as you can at the start of the flight. E.g. Watch 4-5 hours of movies, read, then try to sleep...
Good luck, and do your drinking before you arrive, booze is very dear in Singapore!