I didnt run the grapevine.. but did run I80 last summer in a 40 year old bus with a 392 V8 and an AT-540.. was headed up the 8700 ft grade in wyoming.. the engine was tuned for thinner air so it was hauling ass up the grade with my foot to the floor.. until i got the transmission so hot I boiled out a few quarts of fluid.. it didnt catch on fire and despite the fact I measured WELL OVER 300 with an infrared gun on the trans case, my fluid stayed red... I let it cool down, filled it back up.. and ran it nice N easy at 25 in 3rd gear.. going down I kept it in 3rd and did the "pulse and glide" method of braking... let it run up to about 35.. braked it back to 30.. let it slowly go back up to 35.. etc.. never smelled hot brakes or felt like i was going to have any issues..
well actually at one point I REALLY smelled hot brakes and so I took the nearest ramp which was uphill... stopped only to find it was a Semi ahead of me (I surmise) since he was at the bottom of the hill in a plume of brake smoke when I finally did get there..
taking it slow was the theme here that got me through it.,. when i tried to haul , the bus complained...
I finished the next 1900? miles of the trip without any issues on the transmission so apparently I didnt ruin it