My comments are based on my recent experience with Immigration in Chiang Mai.
Yes, you only need ThB 40,000 per month, but first you have to deposit ThB 300,000 in a fixed account. I lived in Thailand for beginning in 2011 on a retirement visa. Then, the couselate would issue a letter that my U.S. Social Security benefit was a certain amount (greater that 65,000 per month, so I did not have to put 800,000 in the bank.
We then moved to the U.S. after getting my wife a greencard and were there for six years. We returned two months ago and I came in on an exempt 30 day tourist visa as my retirement visa had expired. The first thing that was required is that I deposit 300,000 in a fixed account. But the important thing is that the process was easier because I came back in on the tourist visa. I have been told that generally immigration does not not look kindly on Farang getting to reduce the fixed account amount.
I would also pay a good immigration law firm for an advice consultation. There is also a minimum amount of time you need to have been married and registered. And of course many of the rules change depending on where you are. Good luck.