Help! Slant 6 keeps dying

That is classic tight valves. As long as the revs are up, it will continue to rev, cause there is enough effective compression left to motorvate the car. But as soon as the revs come down, the compression is insufficient and the engine stalls. Let her cool down and everything shrinks and now maybe you have enough compression again to get moving.
If I am right and you continue to operate like this, you will end up with burned valves and that leads to a parked car or a sudden loss of spending money.
So my 2cents is set the valves. This is supposed to be done with an up-to-temp engine, but on a slanty it takes forever to get the cover off and by the time you get to the last cylinder, who knows what the running clearances will be. I set mine in the morning when the engine temp has stabilized overnight. I can take an hour or more, no sweat.
You have to use feelers that are no wider than the valve stem,or you will get an erroneous reading, cuz the stems hammer grooves in the rocker arm;and your feelers will have to fit into the grooves.
The valves have to be set with the lifters on the base circle.
I set mine at about .013/.022 IIRC;Loose is better than tight.