marriage visa, the real requirements

I keep getting different answers on this and it is becoming a bit confusing as no 2 thai immigration oficers can give me a straight answer. I am on a pension and earn over 40,000 baht a month, do I need to have 400,000 baht in the bank as well or not. Also what else do I need to get a marriage visa and how long before my current visa expires do I need to do it. We have asked at the local immigration office a few times now and they either have no idea, tell me to get a retirement visa or tell me I need 400,000 baht in the bank, they have no idea about my monthly payments and cannot even tell me how I am supposed to prove I get what I say. Can anyone here tell me how to prove my monthly pension amount and if I need money in the bank as well, also what other documents etc do I need to show and how long does it take (or how long before my visa expires should I do it), my current visa(12 month multi entry non resident visa with 2 month extension) expires on 19 december and I do not want to do another Ranong run.

Thanks