I currently have an extension based on marriage by use of the 400k in a bank account. Next year I want to use the income route. My income will be from salary received in Thailand.

What are the exact documents relating to income that will be required by immigration for this? And how many months worth of documents/records will I need to produce?

If you have been working long enough to do a annual tax return they will want a copy of it and the 2 or 3 months of tax payment receipts to prove the the 40k baht minimum income.

Dependent upon the office where you apply they may also want some company records from you employer. It would be best to ask your local office what they require,