Small block 727

Redo the 904. 904 is faster than a 727. less parasitic loss.
Yeah, I have been told that, but the trans shop owner was skeptical. He is going to do his research and get back to Me. Ideally, keeping the 904 would save me a lot of other things I would need to change. The car and tranny are 1965 273 v-8 904, cable shift on the column set up, swapping in the 340. I would like the trans to handle up to 375-400 h.p. considering future upgrades of course.
I should also say that I am looking for a 742 or 489 sure grip 3rd member. not so sure the 741 will be adequate.