The e-file system is a trivial exercise compared to calculating the tax return in the first place. An e-file is simply a series of records that replicate the numbers on a properly filled out paper return*. The exact required format of each record is well documented in Pub 1346. Figuring out the numbers to put on that paper return in the first place is the hard part.
I understand what you are saying. I should myself have said "you could implement a complete tax calculation software that prudces 1040 efile format" rather than saying "implementing a 1040 efile". The former is what I really intended to express.
BeanSmart website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.