I don't doubt you. And some of the later '70s smogger motors called for a big whack of initial timing. OP's got a 225, tho, made before then. That's what my 5ish-degrees recommendation was meant for.
I unnerstand. I run 21 initial on Vixen's 225, but she's pretty heavily modified and I have the total limited to 30 degrees. She runs way stronger than an inline six has a right to. We probably put over 100 miles on her today.