I'm rebuilding my 3k engine right now but later this year, we're gonna drop a race spec 4AF engine. what tranny should i use? will the k40/k50 tranny fit the 4AF engine?

The 4-AC uses the T-40 and T-50 so I'm thinking that the 4-AF would the T series box as well

You would need an A-series bellhousing, i.e 3A or 4A.

I think a K50 gearbox did come with a 3A bellhousing k50, but it would be better off having a T50 from a AE70, AE71, or AE85. It should have 4A written on top of the bellhousing.