Upgrading the 904 to a 998 or an 999 (2.47 1st and 1.55 second) with six thin clutches in place of 5 with a custom converter and a Trans-Go TF3 manual valve body kit would make it perform like you want. It's the route I would take if I kept it at a stock stroke. 3.58 inch stroke is also a good option for 367-379 inch combination. I do like a four speed for auto cross though.
After speaking with @replicaracer43 I’m thinking of doing a 4” stroke engine. What trans combination would you suggest if sticking with the 904? I’d like to go 4-speed ideally but incase that doesn’t work out
