If you're asking about how to qualify for ACA tax credits and/or cost sharing reductions, those are based on three things: (1) Your MAGI, (2) your tax household size, and (3) your zip code.
You can always look at your prior year tax returns or fill out a pro forma tax return for 2016 or 2017 to see what your MAGI was or will be. For my state, I can go to my local ACA exchange website, put in my MAGI, and my age and my kids' age, and it will tell me what my ACA subsidies will be.
As you know, for most people their MAGI is just their AGI, which is just the number on the bottom of page 1 of your 1040.
Reducing your 401k distribution should reduce your AGI; whether you can do so depends on the rules of your 401k. Check with your 401k administrator to see if you can reduce your distribution.
Whether reducing your 401k is enough to lower your MAGI to qualify for subsidies depends on your other income and deductions.
ACA subsidies max out at 400% of the Federal Poverty Level. The FPL depends on where you live and your family size. Read here for more details:
https://thefinancebuff.com/federal-poverty-levels-for-obamacare.html
For ACA subsidies, the government currently does not question or look into your assets. They will look into and question you about your taxable income and your household tax size.
Your household tax size is basically the total number of exemptions you claim on the first few lines of your 1040 - line 5 I think it is.
I'd suggest looking at your state exchange website and starting to read about how things work and see what plans are available there. Whether it is affordable to you depends on which state you live in, your age(s), and your income level. Although I am philosophically opposed to the ACA, there are some positives about it in my opinion, and it has let me purchase affordable insurance for my family and in my particular situation. Others have not been as fortunate, so you just need to see how things shake out for you.
Good luck!