Why SCHED 40? SCHED 40 is not legal for physical protection. Needs to be sched 80. If not for physical protection, why are you putting cable in conduit?

You can't use THHN period underground no matter WHAT you encase it in. THHN-PVC tray cable is not usually approved for wet areas (unlike THHN individual conductors which are often dual rated as THWN which is approved for wet areas). Underground is always considered wet (conduit or no).

You seem to be confused on wire sizing issues.

There are several issues. First, you have to size the feeder breaker based on the wire ampacity. That means the feeder breaker can not be larger than 75 amps (modulo some small exception for trade sizes) if you use your #6 cable.

The ampacity of the subpanel does not determine the feeder size. What you need to do is compute what the actual load on the panel is going to be (and that's not just adding up the size of the breakers installed there in most cases).

Your "distance" issues aren't an ampacity issue. This is a voltage drop issue. You indeed may wish to use #4 wire if you have a 70A load at the far end of 75'.

So, the first thing I would do is properly compute the load at the subpanel.

Then I would use that amount of amps plugged into a voltage drop calculator to find the wire size that would provide 3% or less voltage drop at that distance.

Then I would size the breaker to the feeder conductors in use (or the panel ampacity which ever is less).

You can not use the 90°C column of the ampacity table in most cases. The breakers and lugs in the panels are only rated for 75°C at most.

NEC 110.14 (C) (1) (a) (1) and NEC 110.14 (C) (1) (a) (3)

http://freenec.com/T11.html