Accounting methods - cash/accrual

The nature of my work is that a significant amount of my pay comes from overtime and it can sometimes take 2-3 months for the overtime pay to come through from my employer. This creates a situation where I might do some work during one US tax year but not receive the money until the following year.

Most folks on this board would just do their US taxes based on the amounts on their payslips for the calendar year. However, I have calculated that it might be to my benefit to declare my earnings to the IRS in the tax year when I did the work, rather than when I got paid. Does the IRS allow this method of calculation?

Alternatively is it allowable for me to take my P60 that I get at the end of the UK tax year and distribute the earnings on a pro rata basis into the appropriate US tax years? For instance take the P60 for the 2010-11 year and put 9/12 of that amount on my 2010 US return and 3/12 on my 2011 return.

There is some stuff on the IRS foreign tax credit instructions about cash vs accrual methods of accounting, but I wasn't sure if that applied only to self employed people. I know if I were working in the US as an employee I would have to use whatever figures showed up on my W-2 - which would probably be a simple cash calculation.