Mild 340 8 3/4 gear and possibly converter question

Yes, I think 3.55s would be my choice for your build/cam. In the light '65 you may even be happy with 3.23s. If your 904 is a later version with the low 1st gearset, I'd lean even more heavily towards the 3.23s, depending on what you ultimately end up with for tire height- but your current 25s would be a good match. Much taller of a tire would swing me back to the 3.55s