I've put 100,000 miles plus on a 318 , they last if maintained properly . Weak spots were , are the plastic timing chain gear(I have 3 on my wall ), since you have the double roller steel gear you're good to go .
Was mainly concerned about life versus the use of 3.91s with my 904 still in the car. I limit the freeway revs to 3,500 rpm, which still seems acceptable and have increased oil change intervals. Should keep things in good shape.
Hell, the 440-3 in my old '81 C class 19 ft trans van went 96k miles with 4.10s howling at 3500 rpm on the highway with no problems and is still running great to this day in the new owners hands with over 100K on it.